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Influence of the geometry of a hydrogen bond on conformational stability: a theoretical and experimental study of ethyl carbamate. M. Goubet,*a R. A. Motiyenko,a,d F. Real,a L. Margulès,a T. R. Huet,a P. Asselin,b P. Soulard,b A. Krasnicki,c Z. Kisielc and E. A. Alekseevd

Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X 5

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First published on the web Xth XXXXXXXXX 200X DOI: 10.1039/b000000x

Spectra of ethyl carbamate (urethane) in the gas phase have been recorded in the microwave (4 - 20 GHz), millimeter-wave (49 - 118 GHz and 150 – 235 GHz) and mid infrared (1000 – 1900 cm-1) regions. At the same time, high level ab initio calculations have been performed in order to both predict experimental result and help in understanding the physical properties of the system. An extensive set of spectroscopic constants for the two most stable conformers in the gas phase, that might be useful for astrophysical databases, has been derived from the observed signals. The most stable conformer has been undoubtedly identified. Finally, influence of the geometry on a hydrogen bond strength and conformational stability has been discussed on the basis of theoretical and experimental results.

1. Introduction Besides the interest of the carbamate group for drug design and polyurethane polymers, ethyl carbamate (also called urethane, H5C2OC(:O)NH2) is known as a genotoxic carcinogen and has recently been observed in some alcoholic beverages.1 From a chemical-physics point of view, the interest of urethane in the gas phase lies in its structural and conformational composition. The most recent study2 (where all previous studies are listed) combined quantum chemical calculations with microwave spectroscopy at room temperature. The coexistence in the gas phase of two stable conformers was clearly established. They are denoted conformer I and conformer II and are drawn in Figs. 1 and 2, respectively. Experimentally, conformer I was estimated to be more stable than conformer II by about 0.5 kJ/mol. Because of the lack of symmetry in its structure, conformer II is actually a surprising low energy conformer and has not been observed in the crystalline state: only conformer I was detected by X-ray cristallography.3

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The present study of both conformers of urethane includes new series of measurements in the microwave, millimeter-wave and infrared regions together with high level ab initio calculations. We will show that the relative stability of conformer II might be due to a weak intramolecular interaction between the O1 and H8 atoms (see Fig. 2). Therefore, with one conformer with a long range interaction and another one without, urethane appears to be a good candidate to obtain more informations about the structural and energetic properties of weak intramolecular hydrogen bonds. From the results, the conformational stability is discussed in terms of a five members ring interaction and it is compared to higher members ring interactions reported in the litterature. As an application, our study on urethane was also motivated by its possible presence in the interstellar medium. Indeed, it has been recently suggested4 that methyl carbamate

(H3COC(:O)NH2) may be the direct product of a reaction between the two abundant interstellar molecules isocyanic acid (OCNH) and methanol (H3COH). In the same manner, if ethanol, which is also an abundant interstellar molecule, is substituted to methanol, the reaction product would then be ethyl carbamate (H5C2OC(:O)NH2). Consequently, a useful set of spectroscopic parameters is presented.

Fig. 1: Ab initio structure of conformer I of urethane (MP2/aug-cc-pVTZ, 60

see table 1).

Fig. 2: Ab initio structure of conformer II of urethane (MP2/aug-cc-

pVTZ, see table 1).

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2. Experimental and computational details 2.1 Product

A sample of urethane was purchased from Sigma-Aldrich and used without further purification. Under normal conditions, urethane is a white crystalline powder with melting point at 322 K.

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2.2 FTMW spectroscopy

Rotational spectra of urethane were recorded in the frequency range 4 – 20 GHz using the molecular beam Fourier transform microwave spectrometer of PhLAM Laboratory in Lille.5 A heated nozzle was used, which allowed to mix urethane vapours with neon carrier gas at backing pressure of 2.5x105 Pa. A temperature of about 373 K was found to optimize the signal to noise ratio and to prevent decomposition. The mixture was introduced into a Fabry-Perot cavity at a repetition rate of 1.5 Hz. Molecules were polarized within the supersonic expansion by a 2 μs pulse and the free-induction decay signal was detected and digitized into 4096 channels. After transformation of the time domain signal, molecular lines were observed as Doppler doublets. Transition frequency was measured as an average frequency of the two Doppler components and for the most of lines the uncertainty of the measurement is estimated to be 2.4 kHz.

2.3 Conventional absorption spectroscopy

In the experiments with conventional absorption spectrometers in Kharkov and Lille, long pathlength absorption cells working at room temperature with low vapor pressure have been used. Compared to MWFT experiment the sample of urethane was heated only up to 308 K in order to avoid high condensation on cold walls of the cell. A gas pressure of ca 2 Pa, reached under these conditions, was enough to observe rotational lines. The rotational spectra of urethane were recorded in the frequency range 49 – 118 GHz using the millimeter-wave spectrometer at the Institute of Radio Astronomy of NASU in Kharkov6 and in the frequency range 150 – 235 GHz using the millimeter-wave spectrometer of PhLAM Laboratory in Lille.7 The uncertainty of frequency measurement of a single isolated line is estimated to be 10 kHz for the spectrometer in Kharkov and 20 kHz for the spectrometer in Lille.

2.4 Stark effect measurements

Stark effect measurements were made at conditions of supersonic expansion with the cavity FTMW spectrometer in Warsaw. The electric field was applied by using a specially designed set of Stark electrodes8 that are able to produce a field of considerably improved uniformity in relation to standard parallel plates. The electrodes are separated by 27 cm and their effective separation was calibrated using the procedure relying on two different calibration substances, CH3I and CH3CN, which has recently been described in some details.9 The urethane sample was held in a small metal container close to the expansion nozzle, and this container and the surrounding gas handling system were heated to about 363 K. The sample was carried in argon carrier gas at a pressure of

1.2x105 Pa passed through the container, and expanded into the chamber of the spectrometer at a rate of 4 Hz.

2.5 FTIR spectroscopy

The LADIR Laboratory supersonic jet-FTIR spectrometer facility has been described in detail elsewhere.10 An ohmic heating device was needed to generate concentrations of sublimated urethane gas in agreement with the required flow conditions for our continuous supersonic jet. The heating zone was all along the gas injection handling from about 50 cm upstream the collision zone of the jet until the nozzle exit in order to prevent recrystallization of the compound. The device included different heating elements which enable to independently regulate in the 300 - 420 K range both temperatures of the gas line and of the glass cylinder containing crystalline powder of urethane. Practically, urethane/Ar molar ratios up to 2/1 were obtained by flowing argon over the sublimated urethane gas. The mixture was then expanded as a function of the ramp temperature at a total stagnation pressure in the 3x103 - 104 Pa range through a circular nozzle of 1 mm diameter. Jet-cooled urethane was finally probed by the 16-pass arrangement of the IR beam of a Bruker IFS 120 HR interferometer equipped with different bandpass optical filters and focussed on a MCT photovoltaic detector (cutoff above 12 μm). For the series of jet spectra recorded over a large spectral range (1000-1900 cm-1), a band pass filter with a cutoff below 5 μm was used. In the spectral region around 1300 cm-1, where several structured vibrational bands were observed, a narrow band pass filter (250 cm-1 FWHM) was used to improve the signal-to-noise ratio. In our typical conditions of continuous supersonic expansion, the sublimation of 15 g of crystalline urethane enabled to record a series of FTIR spectra at ranged temperatures during about one hour. Each spectrum was the Fourier transform of 50 coadded interferograms at 0.1 cm-1 resolution. During the jet experiments, a large inox plate cooled at 150 K and located in front of the jet expansion has been used to trap urethane, in order to limit at once solid depositions on optics and room temperature absorption due to the vapor pressure of crystalline compound. About 40 % of the initial quantity has been recovered, that was sufficient to prevent pollution on optical elements but not to make negligible the room temperature contribution. Consequently, additional FTIR spectra of urethane at 300 K have be recorded in a 75 cm pathlength cell and subtracted from the jet spectra.

2.6 Ab initio calculations

The potential energy surface of urethane was previously investigated at the MP2/cc-pVTZ and at the B3LYP/6-31G* levels of the theory.2 In order to get a more accurate value for the relative energy of the conformers I and II, the dynamic correlation was computed using the MP2 and the CCSD(T)11 methods. In the numerical investigation, the geometry optimizations and the frequency calculations were performed at the MP2 level. Energy calculations at the CCSD(T) level have been computed using the optimized MP2 geometries. In addition, the transition state (TS) existing between the conformers I and II was also characterized. The calculations

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were conducted using the Molpro2006.112 and Gaussian0313 software packages. The reliability of the ab initio calculations is expected to increase with the size of the basis set. Standard double, triple and quadruple valence basis sets (VXZ, X=D,T,Q,...) and correlation-consistent polarized valence basis sets (aug-cc-pVXZ) built by Dunning and co-workers,

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14-16 were employed to describe the hydrogen, carbon, oxygen and nitrogen atoms. The cardinal number X was used to extrapolate the correlated energy in a complete basis set (CBS).17,18 Considering the calculation cost with respect to the higher values of X (Q,5) used for the electron correlation treatment, the resolution of the identity (RI) approximation of the MP2 level (RI-MP2)19 was used to compute the energies and the geometries.20 The results were compared to those obtained with the MP2 method for X=D,T. In order to compute the RI procedure, the RI and the auxiliary basis sets21 available in the Turbomole package 22-23 were employed. In a final step, the frequency calculation was performed for the cc-pVDZ, cc-pVTZ, aug-cc-pVDZ and aug-cc-pVTZ basis sets and the corresponding vibrational zero-point energy (ZPE) was estimated. The transition state energy, also ZPE corrected, and structure were calculated at the MP2 level with the aug-cc-VTZ basis set using the QST3 procedure as implemented in the Gaussian03 package. Finally, the G3 compound method was used to calculate the relative energy associated with conformer I and conformer II.

3. Results and analysis 3.1 Computational study

At first the objective of our ab initio calculations was to validate the equilibrium structure obtained previouly.2 In order to support the discussion on a possible intramolecular interaction in conformer II, basis sets augmented with diffuse functions (aug-) have been prefered. Our best geometrical parameters, obtained at the MP2/aug-cc-pVTZ level for the conformers I and II are summarized in table 1. They are very similar to those calculated in the previous work with the cc-pVTZ basis set, except a variation of about 1° for the O1-C2-N6-H13 angle.2 The angular and the distance variations associated with the results obtained with the aug-cc-pVDZ basis set are also negligible (0.1° and 0.02 Å respectively). Therefore, we can assume that the geometry is unchanged for higher basis set quality. The second objective of our ab initio calculations was to obtain an accurate value of the relative energy between the conformers I and II of urethane. The computed results are reported in table 2. The negative values tend to indicate that the conformer II is more stable than the conformer I. However, an opposite conclusion would appear considering that (i) convergence does not seem to be reached in the MP2 and CCSD(T) calculations: the absolute value of the relative energy decreases with the increasing basis set quality ; (ii) the ZPE corrections effect on the relative energy is about 0.40 kJ/mol and tends to yield an equilibrium between to the two conformers (see values in parentheses in table 2). This highlights the fact that the ZPE correction cannot be neglected. Moreover, calculations at the G3 level show a

conformer I more stable than conformer II by +0,33 kJ/mol.

Table 1: The MP2/aug-cc-pVTZ molecular structure of the conformers I and II of urethane.

Bond lengths/pm

I II Bond lengths/pm

I II

C2-O1 121.4 121.5 C4-C5 150.8 151.2

C2-O3 135.4 135.4 C5-H9 108.9 109.0

C2-N6 136.6 136.5 C5-H10 108.8 108.8

C4-O3 144.2 144.3 C5-H11 108.8 108.9

C4-H7 108.9 108.7 N6-H12 100.4 100.4

C4-H8 108.9 108.8 N6-H13 100.4 100.4

Bond angles/° I II Bond angles/° I II

O1-C2-C3 124.7 125.0 C4-C5-H9 109.6 109.7

C2-O3-C4 113.9 114.5 C4-C5-H10 110.7 110.3

O3-C4-C5 106.9 111.0 C4-C5-H11 110.7 110.5

O3-C2-N6 109.9 109.8 C2-N6-H12 115.8 116.0

O3-C4-H7 108.7 104.2 C2-N6-H13 118.1 118.3

O3-C4-H8 108.8 108.8

Dihedral angles/°

I II Dihedral angles/°

I II

O1-C2-O3-C4 1.0 -1.1 O3-C4-C5-H9 -179.9 -176.7

C2-O3-C4-C5 179.8 -82.5 O3-C4-C5-H10 60.4 63.9

C4-O3-C2-N6 -177.2 -179.1 O3-C4-C5-H11 -60.2 -57.1

C2-O3-C4-H7 58.6 157.2 O1-C2-N6-H12 16.3 15.7

C2-O3-C4-H8 -58.9 40.3 O1-C2-N6-H13 164.7 165.5

Table 2: relative energies of conformer II with respect to conformer I of urethane (kJ/mol) computed at different levels of the theory. The ZPE corrected energies are reported between parentheses.

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MP2 RI-MP2

CCSD(T)//MP2 G3

cc-pVDZ -1.26 (-0.59)

- -

cc-pVTZ -0.59 (-0.13)

- -

cc-pVQZ -0.71 - -

aug-cc-pVDZ -0.88 (-0.42)

-0.84 -0.79

aug-cc-pVTZ -0.42 (-0.04)

-0.42

-0.29

aug-cc-pVQZ - -0.42 -

aug-cc-pV5Z - -0.42 -

CBS -0.54

- +0.33

One should then notice that the very small energy

difference values are within the computational errors, of the order of several tenths of kJ/mol. As a consequence, checking the convergence of our energy calculations is a critical issue. The CBS extrapolation done at the RI-MP2 level, with converged values, yields an identical energetic scheme and does not change the picture. Indeed, with the RI-MP2 electron correlation method and higher basis set quality, the relative energy is not modified from X=T to 5. Therefore, the MP2/aug-cc-pVTZ combination is thought to offer the best compromise between calculations time and accuracy of results, both in structure and in energy. In summary, calculations alone are not able in this case to point out which conformer of urethane is the most stable but only that they are very close in energy.

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Finally, the barrier between the two conformers is relatively low. A transition state, with only one imaginary frequency, was found with an energy above conformer I of 3.64 kJ/mol at the MP2/aug-cc-pVTZ level (3.89 kJ/mol with the ZPE correction and after subtraction of the frequency in conformer I corresponding to the imaginary one in TS). The structure of the transition state is characterized by the rotation of the ethyl group along the O3-C4 bond with a C2-O3-C4-C5 dihedral angle value of -128.5°. The calculated values of the spectroscopic parameters, dipole moment components and harmonic frequencies are reported in section 3.2 and 3.3 along with the corresponding experimental results.

3.2 Rotational spectroscopy

Rotational spectrum The conventional microwave spectrum of urethane reported in the literature2 was recorded in the 16.5 - 56.0 GHz region at room temperature. A few lines associated with the ground vibrational state of both conformers and with the lowest excited O3-C4 torsional mode (ν1) of conformer II were observed and assigned, leading to a limited set of spectroscopic parameters (principal rotation and quadratic centrifugal distortion constants). The spectra recorded with the Lille molecular beam FTMW spectrometer allowed to observe the nuclear quadrupole hyperfine structure of the rotational lines, due to the 14N nucleus (nuclear spin value of I=1), and to evidence many new lines with low quantum numbers values, at a rotational temperature of about 3 K. A typical example of signals, associated with the ground states of conformers I and II, is displayed Fig. 3. With these new results, it turned out that R-type transitions of conformer I observed in the previous study2 had to be reassigned, leading to a new set of principal rotational constants. The spectra recorded in the millimeter-wave range exhibit typical b-type transitions with features such as bQ1,-1 bands. For some low-Ka rotational transitions, we were able to resolve partially the nuclear quadrupole hyperfine structure. In total in the millimetre-wave range, frequencies of 1045 lines corresponding to 457 rotational transitions of conformer I ground state and of 1382 lines corresponding to 578 rotational transitions of conformer II ground state have been measured. The maximum values of quantum numbers of assigned transitions are J = 73 and Ka = 17 for conformer I

and J = 83 and Ka = 22 for conformer II. All the frequencies measured in the mm-wave range were fitted together with FTMW measurements in order to produce the final set of rotational constants. To minimize the standard deviation of the fit, unresolved hyperfine components were fitted taking into account their relative intensities. Results of urethane microwave spectra analysis are presented in Tables 3 and 4.

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Fig. 3: composite FTMW spectrum of the JKaKc = 111-000 rotational transitions of conformers I and II of urethane. The hyperfine structure is

shown in detail.

The sets of parameters were obtained by least squares fit of the FTMW and conventional absorption spectroscopy data. A global least-squares fitting program (SPFIT, developped by H. Pickett, Jet propulsion Laboratory)

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24 was used to fit the mesured transitions to a semi-rigid rotor Hamiltonian, i.e. a standard asymmetric-top Hamiltonian with a Watson A-reduction in the Ir-representation. The principal rotational constants (A, B, C) were obtained, as well as the quadratic (Δ, δ), sextic (H, h) and octic (L,l) centrifugal distotion parameters. The nuclear hyperfine structure of the rotational lines was modeled with the χii diagonal components of the nuclear quadrupole hyperfine coupling tensor, taking into account that χaa + χbb + χcc = 0.25

The agreement between experimental and calculated parameters is qualitatively very good (signs and orders of magnitude of constants) and quantitatively acceptable (less

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than 15% of error except for δK and χaa in conformer II).

Table 3: spectroscopic constants for conformer I of urethane.

v=0 v1=1 v2=1

A/MHz 8989.50712(13) 8984.4 a 8936.13594(20) 8976.061(65)

B/MHz 2136.621931(27) 2148.1 a 2136.747260(53) 2134.7454(56)

C/MHz 1766.526182(27) 1773.0 a 1770.725476(51) 1766.03290(78)

ΔJ/kHz 0.178200(13) 0.176 a 0.186067(30) 0.1965(17)

ΔJK/kHz 1.236546(96) 1.275 a 1.09795(15)

ΔK/kHz 5.2607(13) 5.260 a 6.06177(71)

δJ/kHz 0.0346656(32) 0.035 a 0.0339900(42) 0.0454(10)

δK/kHz 0.64648(18) 0.709 a 0.42428(58)

HJ/mHz 0.0258(23) 0.0413(51)

HJK/Hz -0.00127(11) -0.00226(24)

HKJ/Hz -0.03095(39) -0.04806(82)

HK/Hz 0.0403(37) -

hJ/mHz 0.01064(81) -

hJK/mHz -0.743(82) -1.73(18)

hK/Hz 0.0218(27) 0.0418(64)

χaa/MHz 2.1151(14) 2.06 a 2.1151b

χcc/MHz -4.2818(15) -4.15 a -4.2818b

N 1165 577 30

σ/MHz 0.0149 0.0169 0.022

a Calculated at the MP2/aug-cc-pVTZ level. b Values fixed to the fundamental state ones.

Besides ground states of both conformations, the rotational spectra of the ν

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1 and ν2 vibrational states were assigned on the basis of mm-wave spectra analysis. The ν1 mode is associated with the O3-C4 torsional motion, and its harmonic frequency value was calculated at 68.9 and 94.1 cm-1 for conformer I and II, respectively. Similarly, the ν2 mode is associated with the C2-O3 torsional motion, and its harmonic frequency value was calculated at 138.1 and 114.5 cm-1 for conformer I and II, respectively (see section 3.3). For conformer I, frequencies of 577 lines of ν1, corresponding to 293 rotational transitions with maximum values of quantum numbers J = 72 and Ka = 17 have been assigned and fitted. For ν2, the intensity of the rotational spectrum is much weaker, and only 30 lines could been assigned providing the determination of principal rotational constants and only 2 quartic distortion parameters.

Table 4: spectroscopic constants for conformer II of urethane 20

v=0 v1=1 v2=1

A/MHz 7565.418436(75) 7571.7 a

7551.71088(48) 7606.09495(65)

B/MHz 2414.784339(23) 2439.6 a

2397.83112(11) 2407.22157(20)

C/MHz 2116.375044(27) 2127.9 a

2105.344828(86) 2115.79653(11)

ΔJ/kHz 0.916116(15) 0.839 a

1.164396(82) 0.80868(16)

ΔJK/kHz 0.386058(86) 0.330 a

1.5003(13) -0.1613(19)

ΔK/kHz 12.56420(63) 11.638 a

11.0493(43) 14.6518(60)

δJ/kHz 0.0837883(48) 0.076 a

0.120708(36) 0.060010(76)

δK/kHz -3.49003(40) -2.202 a

-7.9644(34) -2.2624(49)

HJ/mHz -1.3236(30) -1.729(19) -2.745(57)

HJK/Hz -0.001802(95) -0.0279(11) 0.0138(22)

HKJ/Hz -0.10742(43) -0.3413(97) 0.213(12)

HK/Hz 0.1058(11) 0.418(13) -0.209(18)

hJ/mHz -0.7398(11) -0.5809(97) -0.795(28)

hJK/Hz 0.08468(23) 0.16195(97) 0.1512(17)

hK/Hz 0.7697(20) 0.227(32) 1.494(63)

LJJK/mHz -0.0006489(35) - -

LKJ/mHz 0.003879(61) -0.0326(41) -

LKKJ/mHz -0.01152(53) - -

lJK/mHz 0.001658(36) - -

χaa/MHz 1.8923(11) 1.49 a

1.92(21) 1.73(31)

χcc/MHz -3.7841(11) -3.24 a

-3.75(12) -3.72(17)

N 1521 343 236

σ/MHz 0.0137 0.0155 0.0166

a Calculated at the MP2/aug-cc-pVTZ level.

For conformer II, both excited states were found to be perturbed at high quantum numbers values (J > 50, Ka > 10 for ν1 and J > 45, Ka > 8 for ν2). These two states are probably linked by a Coriolis interaction, not detailled in the present study. The spectroscopic parameters in table 4 were obtained for the unperturbed levels.

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Electric dipole moment The dipole moment for conformer I was determined from Stark effect measurements on completely resolved Stark lobes of hyperfine components of the nuclear quadrupole hyperfine structure due to the 14N nucleus. Two different bR-type rotational transitions were used, 111-000 at 10755 MHz and

212-101 at 14290 MHz. Stark shifts were measured for 10 different MF lobes of 5 different hyperfine components. The measurements are summarized graphically in Fig. 4, in which it is possible to discern several nonlinearities relative to simple second order Stark behavior. 5

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Fig. 4: summary of Stark effect measurements made for two different rotational transitions of conformer I. Circles denote experimental points and continuous lines are predictions made on the basis of the final dipole

moment fit. 10

Table 5: comparison of measured and calculated values of dipole moment components for conformer I and II of urethane.

Conformer I Conformer II

Exp. Calc.a Exp.b Calc.a

μa (D) 0.5878(20) 0.65 - 0.01

μb (D) 2.2579(21) 2.48 - 2.25

μc (D) -c 0.67 - 1.11

Nlinesd 23 -

σfit(kHz)e 2.89 -

a Calculated at the MP2/aug-cc-pVTZ level. b Not observed experimentally (see section 4.1). c Not determinable in the ground vibrational state (see text). 15

d The number of fitted Stark measurements. e Deviation of fit made with the QSTARK program, in which rotational, centrifugal and hyperfine constants were fixed at values from Table 3.

Such nonlinearities are rather common in the relatively low electric field regime of measurements made with cavity FTMW spectrometers, where the magnitudes of the hyperfine splitting and the Stark splitting are comparable. This intermediate field regime is, however, satisfactorily tractable by setting up and diagonalisation of the complete Hamiltonian matrix separately for each unique pair of values of M

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F and of the applied electric field. A treatment of this type is embodied in program QSTARK26 and the results of fitting the present measurements with this program are given in Table 5. Ab initio calculations for conformer I predict that all three dipole moment components will be non-zero, but it was only possible to determine μa and μb from the measurements. The third

component, μc, is calculated to be about 0.7 D in the equilibrium configuration, and will arise almost entirely from the NH2 group. This group is subject to a low-barrier inversion motion across the ab inertial plane so it is not surprising that μc is effectively zero in the ground state, due to averaging over the double-minimum large-amplitude motion. Furthermore, although the μa component is rather small, there was sufficient sensitivity of Stark shifts for the rather strong b-type transitions to satisfactorily determine its value. The agreement between experiment and calculation is acceptable and only consistent with molecular geometry of conformer I. Concerning conformer II, any of the lines have been observed on the Warsaw spectrometer for a reason that is discussed in section 4.1.

3.3 Vibrational spectroscopy

Figure 5 displays a sample jet-FTIR spectrum of urethane recorded around 1300 cm-1 with a narrow band pass filter. The contribution of the room temperature absorption of crystalline urethane deposited in the jet chamber is here subtracted but it finally makes little difference on the vibrational band contours of the jet-cooled spectrum.

Fig. 5: detailed view of the 1230-1460 cm-1 region of the jet-FTIR

spectrum of urethane. The jet-cooled contribution has been obtained by 55

subtracting the 300 K contribution recorded in the cell.

Bands assignments of the jet-FTIR spectra are proposed on the basis of ab initio harmonic calculations for both conformers (see Table 6). Attributions (reported in the last column relative to each conformer) are guided by theoretical values of both harmonic frequencies and band intensities. For example, the three most intense bands observed at 1333, 1381 et 1410 cm

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-1 are assigned to the ν19, ν20 and ν21 vibrational modes, respectively, from the following arguments: (i) theoretical frequencies for these three vibrations are larger than the experimental ones by only 20 - 30 cm-1; (ii) calculated intensity ratios for the three-band pattern ν19/ν20/ν21 are very close to the experimental integrated band intensities (14/2/1 theoretically vs. 14/2/0.5 experimentally). Diagonal anharmonic constants for relevant vibrational modes have been calculated within the adiabatic approximation27 in order to estimate the frequencies more closely, but the correction on each harmonic frequencies is only of a few wavenumbers. Thus, ab initio harmonic frequencies have been

kept for comparison.

Table 6: comparison between ab initio harmonic frequencies of conformers I and II and experimental values derived from jet-FTIR proposed assignments.

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Conformer I Conformer II

Calc. a Exp. Calc. a Exp.

n νn (cm-1) I (km/mol) νn νn (cm-1) I (km/mol) νn

1 68.9 0.8 94.1 6.1

2 138.1 7.8 114.5 1.6

3 204.4 7.9 223.3 3.2

4 267.8 1.1 328.7 10.6

5 367.9 236.3 345.3 248.6

6 376.0 7.4 395.2 1.0

7 471.3 5.7 486.3 4.8

8 515.3 8.9 520.5 11.5

9 665.1 4.5 661.9 2.7

10 785.4 15.6 785.7 15.1

11 820.1 0.0 801.3 8.6

12 869.1 12.9 860.4 10.3

13 1008.4 18.7 971.2 12.5

14 1102.6 12.5 1099.4 17.1

15 1126.7 140.7 1078 1127.8 31.6

16 1154.9 19.6 1136.1 111.3 1107

17 1191.0 3.2 1205.4 15.6

18 1309.6 1.0 1342.3 20.0

19 1353.0 425.1 1333 1358.3 353.8 1333

20 1412.4 62.0 1381 1408.7 54.2 1381

21 1439.0 16.2 1410 1431.9 25.5 1410

22 1506.9 6.4 1504.9 9.1

23 1519.2 3.2 1511.1 9.4

24 1538.8 6.6 1525.7 8.4

25 1623.2 119.4 1580 1621.8 121.0 1580

26 1806.9 410.2 1773 1805.3 392.1 1770

27 3081.5 12.5 3077.8 13.1

28 3098.7 14.8 3113.9 25.2

29 3150.4 4.3 3164.4 13.5

30 3172.9 14.5 3171.5 5.0

31 3181.8 23.2 3190.7 15.0

32 3628.6 52.3 3630.4 54.7

33 3766.0 65.5 3769.1 67.2

a Calculated at the MP2/aug-cc-pVTZ level.

Although the two conformers should exist according to the computational study, the small differences (a few cm-1) between predicted vibrational frequencies of conformers I and II (see Table 6) make difficult to undoubtedly prove their dual presence, especially in the case of broad and unstructured absorptions as observed here. However, an exception concerns the assignment of the bands observed at 1078 and 1107 cm-1 with intensity ratios estimated to 1/1.5. These frequencies could correspond to the ν15 and ν16 bands of each conformer but theoretical intensity ratios vary between 7/1 (conf. I) and 1/3.5 (conf. II), which largely deviate from the ratio observed experimentally. Another possibility would be to assign the 1078 cm-1 band to the ν15 vibration of conformer I and the 1107 cm-1 band to the ν16 vibration of conformer II. In this case, agreement between theoretical (1.3/1) and experimental (1/1.5) intensity ratios is much better, what supports our assignment and the fact that both conformers are observed in the jet-FTIR experiments.

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Fig. 6: comparison between jet-cooled and calculated contours of the ν19 25

(a) and ν26 (b) bands of urethane. The simulation of the ν19 comprises a transition of one single conformer of A-type with rovibrational coupling

constants αA19 = 0.003 cm-1 and αB

19 + αC19 = -0.002 cm-1. Two

simulations of the ν26 band have been performed: one with a single B-type transition, the other with two B-type transitions apart from 3 cm-1 (one for 30

each conformer). For both cases coupling constants are set to zero.

Concerning the two intense features assigned to ν19 and ν26 vibrational modes, extensive band contour simulations of have been performed in order to determine whether these band patterns could result from an unique vibrational transition or from the overlapping of two vibrations close in frequency.

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Theoretical predictions of the band type and rovibrational coupling constants have been included in these simulations. Results shown in figure 6 do not allow definitive assignments but some tendencies can be proposed. About the ν19 band (Fig. 6a), it clearly appears that the strong asymmetry at the low frequency side (assigned to a P branch) could not be reproduced with a single vibrational transition. About the ν

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26 band (fig. 6b), the scenario of two transitions very close in frequency, one for each conformer, has been preferred. Indeed, the observed typical B-type band pattern (absence of a Q branch with wide P and R branches) is preserved as much as the frequency difference between the conformers band centers is not larger than 3 cm-1.

4. Discussion 4.1 conformational stability and relaxation in the jet

Although ab initio calculations cannot identify the most stable conformer, there are clear experimental evidences pointing out that the conformer I of urethane is more stable than conformer II. First, an energy difference of about +0.50 kJ/mol have been estimated by the authors in reference [2] on the basis of relative intensity measurements. One should note that this value is in qualitative agreement with the calculations at the G3 level from this work giving a value of +0.33 kJ/mol. Second, the study in the crystalline state showed that only conformer I is preferred, with an arrangement in planar layers.3 In general, although not always, only the conformer of minimum energy in the gas phase is kept during the crystallization process. So, unless urethane is a singular case, conformer I should be the most stable in gas phase. Finally, understanding why conformer II has not been observed in Warsaw experiments appears actually to be the clearest evidence. The main difference with Lille experiments is the carrier gas (Ar at Warsaw and Ne at Lille). The heavier is the carrier gas, the stronger is, during the expansion, the relaxation to the most stable conformer, for molecules with a relatively low barrier to internal rotation. This is a well known effect, which has been studied in the litterature. By comparison with ethyle formate and ethanol,28 which have approximately the same energy difference between conformers (about 40 cm-1) and the same barrier to interconversion height (about 350 cm-1) as urethane, almost complete relaxation to conformer I seeded in a Ar jet but not in a Ne jet seems obvious. It allows us to establish unambiguously that conformer I is the lowest energy conformer of urethane. According to this, one can wonder why conformer II is nevertheless observed in Paris experiments, with urethane seeded in Ar as well. As already observed in recent works about furan-HCl29 and thietane-HF complexes, the typical conditions used in the continuous supersonic jet, i.e. low reservoir pressures (around 104 Pa) and poorly diluted gas mixtures in Ar or He (more than 20 %), are much less efficient for the conformational relaxation than those currently used in microwave experiments (reservoir pressures of several 105 Pa and dilutions of about 1 %). In the case of small

interconversion barriers (less than about 4 kJ/mol) between two conformers separated by a small energy difference (less than about 4 kJ/mol), some important differences between the relaxation effects could be observed depending on the nature of the supersonic jet. For example in the case of thietane-HF, both conformers with similar ratios of intensity between the transitions are observed in jet-cooled FTIR spectra when argon is the carrier gas. However, with supersonic expansions used in the microwave experiments, both forms are observed with He as carrier gas but only the most stable form is observed with a heavier relaxing gas such as Ar. The energetic case of conformational isomers of urethane is quite similar to that of hydrogen bonded complexes previously cited. Consequently, in the jet-cooled conditions for infrared spectroscopy, it is expected to observe both forms of urethane whatever is the nature of the carrier gas.

4.2 energy stabilization by an intramolecular hydrogen bond

From simple symmetry considerations, conformer I is easily expected as being one of the most stable conformations with its planar skeleton (geometry is in Cs group excluding the two hydrogen of the amide group), syn orientation of the oxygen-ethyl group compared to carbonyl group and anti orientation of the methyl group compared to O3-C4 bond (see fig. 1). In the same manner, another symmetrically expected conformer would have been a planar skeleton with an orientation of the oxygen-ethyl group compared to carbonyl group of 180° from syn, but this conformation has been calculated at about 29 kJ/mol over conformer I. Therefore, it is surprising from a symmetry point of view to have conformer II such as a low energy geometry. Methyl group is in this case out of the plane formed by the other heavy atoms with a dihedral angle of 97.5° from anti orientation where about 120° is expected. Although this deviation in structure has been attributed to steric interactions,30 these interactions do not explain the effect of lowering the energy down to a value very close to the one of the lowest conformer. An intramolecular hydrogen bond between H8 and a lone pair of O1 is thus strongly suggested. Indeed, a comparison between the highest occupied molecular orbitals of the two conformers from our ab initio results shows a significant difference. The HOMOs (a), HOMOs-1 (b) and HOMOs-2 (c) of conformers I and II are drawn in figure 7. In both cases, the HOMO and the HOMO-1 are very similar: the electrons are located in the same region around the nuclei. However, large change appears in the HOMO-2. HOMO-2 of conformer II reveals a long-range interaction (shown by an arrow in fig. 7c) between the bounding orbital of the rotated CH2 group and the lone pair of the oxygen O1, which most likely stabilizes the conformer. Another argument comes from the IR study. The band contour analysis of the spectrum has shown that the observed signal should include two transitions attributed to C2=O1 stretching mode (ν26 band), one from each conformer, separated by a few wavenumbers. Since the changes in geometry between the conformers I and II happen away from these two atoms, and because the C2=O1 stretches are not coupled to other movements in both conformers, the slight red shift of the conformer II ν26 band might be the signature of a

weak intramolecular hydrogen bond. This assumption is supported by results on longer carbon chains like small protected dipeptides. In a study of Ac-Gly-Phe-NH2,31 the authors pointed out several stable conformations in the gas phase with different types of intramolecular hydrogen bonds. The C=O Ace stretch is of particular interest to be compared with our results on the ν

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26 band. The calculated wavenumber of the CO Ace stretch free of interaction (in the denoted βL-γL(g-) conformation) is red shifted of 15-20 cm-1 when involved in a ten (C10) or seven (C7) members ring interaction (β−turn II'(g+) or γL-γL(g-) conformations, respectively). This red shift goes down to 1 cm-1 when involved in a weaker five (C5) members ring interaction (βL-γL(a) conformation). Using such a terminology, conformer II of urethane would have a five members ring interaction, so an expected red shift of the CO stretch of a few wavenumbers, that is the result of our band contour simulation.

This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 9

Fig. 7: HOMO (a), HOMO-1 (b), and HOMO-2 (c) of conformer I and II of urethane. 20

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However, it is usually admitted that the proton donor hydrogen should be along the axis of the proton acceptor lone pair for a hydrogen bond to occur. Because of the SP2 hybrid orbital of O1, a value of 120° for the C2-O1-H8 angle would be expected, instead of our calculated value of about 80°. But, as a general rule, the stronger is the interaction, the larger is the shift. Therefore, a shift of a few wavenumbers of the acceptor vibration mode should be the fingerprint of a weak hydrogen bond, i.e. several tenth of kJ/mol. As an example, in the case of another small protected peptide N-Ac-Phe-NH2,32 the authors have estimated the strength of a C5 interaction at about 0.6 kJ/mol. These literature results together with the results from the present study on urethane lead to two comments. Firstly, this emphasizes the fact that even if the

molecule is not flexible enough for a hydrogen atom to lie in front of an acceptor lone pair, it does not mean that an intramolecular interaction is impossible but rather that the interaction will be much weaker than usually expected. In other words, not only the distance between patterns influences a hydrogen bond strength, but also the orientation of the bonded hydrogen compared to the lone pair of the acceptor. Secondly, such a weak interaction explains the low relative energy of conformer II with respect to conformer I even if it was not expected from symmetry considerations. At the same time, this interaction is not strong enough to observe conformer II at a lower energy than conformer I (without interaction) as it is usually the case.

5. Conclusion An extensive set of spectroscopic constants, for the two most stable conformers of urethane in the gas phase, has been derived from experimental observations in a wide frequency range going from microwaves to mid infrared. Ab initio calculations have been performed in order to both predict experimental results and help in understanding the physical properties of the system. Their overall good agreement with experiments has led to the following conclusions. Firstly, theoretical results are reliable for validation and interpretation of the experimental parameters. Secondly, although a medium level of calculations (MP2/aug-cc-pVDZ) is enough to correctly predict the structures, a higher level with ZPE correction is definitely needed to estimate the relative energies between conformations, and still an error of several tenth of kJ/mol has to be expected. Relaxation process in a seeded molecular expansion has shown that conformer I is undoubtedly the most stable conformer, bringing the controversy about conformational stability to an end. Finally, the mixing of calculated HOMOs and the IR band contour analysis have shown the presence of an intramolecular hydrogen bond in conformer II, for which properties have been discussed. In one hand, such an interaction could explain why conformer II is one of the lowest conformers although there are no clear symmetrical reasons for this. On the other hand, it has been suggested that even if the bonded hydrogen atom does not lie along the axis of an acceptor lone pair, an interaction is still possible but the farther is the hydrogen from linear orientation, the weaker is the bond strength.

Acknowledgments The authors would like to thank Prof. H. Mollendal and Dr. J. Demaison for helpful discussions and Mr J. Habinshuti for recording part of the microwave spectrum. This work was supported by the French ANR-05-BLAN-0091 grant. The authors also acknowledge financing from the Polish Ministry of Science and Technology (grant n° N-N202-0541-33), the PEPCO-NEI network (project n° 509031H) and the INTAS network (YSF grant n° 06-1000014-5984).

Notes and references

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a Laboratoire de Physique des Lasers, Atomes et molécules, UMR 8523, Université des Sciences et Technologies de Lille 1, CNRS, F-59655 Villeneuve D'Ascq Cedex, France. b Laboratoire Dynamique, Interactions et Réactivité, UMR 7075, Université Pierre et Marie Curie, CNRS, case 49, 4 place Jussieu, F-75252 Paris Cedex 05, France.

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c Institute of Physics, Polish Academy of Sciences, Al. Lotnikòw 32/46, 02-668 Warszawa, Poland. d Institute of Radio Astronomy of NASU, Chervonopraporna 4, 61002 Kharkov, Ukraine. * Corresponding author: manuel.goubet@univ-lille1.fr1 D. W. Lachenmeier, Anal. Bioanal. Chem., 2005, 382, 1407. 2 K.-M. Marstokk, H. Mollendal, Acta Chemica Scandinavica, 1999, 53,

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