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Vibrational Spectroscopy 55 (2011) 126–131

Contents lists available at ScienceDirect

Vibrational Spectroscopy

journa l homepage: www.e lsev ier .com/ locate /v ibspec

nfrared spectroscopy of 2-pyrrolidinone and its hydrogen bonded dimers in aold (8 K) inert gas matrix

rasenjit Pandey, Amit K. Samanta, Biman Bandyopadhyay, Tapas Chakraborty ∗

epartment of Physical Chemistry, Indian Association for the Cultivation of Science, 2A & B Raja S.C. Mullick Road, Jadavpur, Kolkata 700032, West Bengal, India

r t i c l e i n f o

rticle history:eceived 27 July 2010eceived in revised form0 September 2010ccepted 30 September 2010vailable online 8 October 2010

a b s t r a c t

Matrix isolation–FTIR spectra of 2-pyrrolidinone monomer and its two hydrogen bonded dimers arerecorded in N2 matrix at 8 K temperature and in CCl4 solution at variable temperature within the range30–45 ◦C. The transient singly hydrogen bonded dimer (SHBD) is identified in the cold N2 by comparingthe matrix spectrum with that in CCl4 solution. The spectral shifts of both amide-I and amide-A bandsindicate cooperative strengthening of hydrogen bonds in doubly hydrogen bonded dimer (DHBD). Density

eywords:atrix isolation

TIR-Pyrrolidinoneransient

functional theory at B3PW91/6-311++G** level of calculation is found to be in good accord to the observedspectral features.

© 2010 Elsevier B.V. All rights reserved.

mide-Imide-A

. Introduction

2-Pyrrolidinone (2PYD, also known as �-butyrolactam) is aiologically important compound, and is used as a precursor ofrospective drugs for treatments of hypertension, sea sicknessnd different pathologies such as cancer [1]. The compound haseceived considerable attention in recent years as a simple proto-ype of cyclic peptide for studies by various spectroscopic methods.imeric complexes generated via hydrogen bonding at its amide

–C(O)–N(H)–) interface also mimic base pairing in nucleic acids,nd a lot of studies were aimed to establish the state of asso-iation of the compound in nonpolar solutions [2–10]. Recently,met et al. used nonlinear dielectric spectroscopy to study the self-ssociation of the compound in benzene using concentrations up to.9 M [10]. The measurement estimated the value of dipole momentf the dimer to be 2.30 D. But this contrasts with the expectedipole moment for a symmetric doubly hydrogen bonded dimericDHBD) species, which is expected to be nearly zero. The authorsxplained the discrepancy by invoking rapid structural intercon-

ersion between open and closed forms of the dimer. However,sing conventional spectroscopic methods it is difficult to get direct

nformation about transition association complexes in the solu-ion phase. Such transient species (e.g., SHBD) can, however, be

∗ Corresponding author. Tel.: +91 33 2473 4971x470; fax: +91 33 2473 2805.E-mail address: pctc@iacs.res.in (T. Chakraborty).

924-2031/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.vibspec.2010.09.013

generated in a controlled manner and kept frozen over consider-able period for spectroscopic studies in cold inert gas matrixes. For2PYD, a matrix isolation–FTIR spectrum was reported earlier byColeman III and Gordon [11], but no detailed spectral analysis wasattempted. In the present study, we have performed a comprehen-sive analysis of the FTIR spectra of 2PYD and its two dimers (SHBDand DHBD) in a cold (8 K) N2 matrix. The FTIR spectra in CCl4 solu-tion were also recorded at a series of temperature to study the equi-librium between monomer and H-bonded dimers. A comparisonbetween the temperature dependent changes of spectra recordedin CCl4 solution and N2 matrix reveals that the SHBD is indeed atransient species. Electronic structure calculations have been per-formed to complement and interpret the measured spectra.

2. Materials and methods

2.1. Experimental

2-Pyrrolidinone (99% purity) was purchased fromSigma–Aldrich and purified further by vacuum distillation.The vapor of 2-pyrrolidinone at room temperature (22 ◦C) wasmixed with high pure N2 (99.99%) in a mixing chamber. The

gas mixture was deposited onto a KBr window attached on thecold finger (8 K) of a closed cycle He cryostat (Advanced ResearchSystems, Inc.). The temperature of the cold finger was controlled bya temperature controller (Lake Shore Model 331). In the gas mixingchamber, the concentration of 2-pyrrolidinone was varied by

P. Pandey et al. / Vibrational Spectroscopy 55 (2011) 126–131 127

F (DHBD

cstst8sowp(Awsdb

ig. 1. Optimized structures of: (a) monomer, (b) doubly hydrogen bonded dimerFT/B3PW91/6-311++G** level of theory.

hanging partial pressure of N2. The optimum concentration of theample in the gas mixing chamber was such that before annealing,he intensities of dimer bands compared to that of monomer weremall. To anneal the matrix, temperature was slowly increased upo 25 K, kept there for half an hour and then again cooled back toK. For solution phase study at room temperature, a home-made

olution cell consisting of a pair of KBr windows and a teflon spacerf 0.2 mm thickness was used. Spectra at different temperaturesere recorded by a home-made variable temperature cell ofath length 1.5 mm. The FTIR spectra were recorded in the mid-IR4000–400 cm−1) region using a spectrometer resolution of 1 cm−1.

commercial FTIR spectrometer (IFS66/S, Bruker Optics) equippedith deuterated triglycine sulfate (DTGS) detector and KBr beam

plitter was used. The spectrometer was purged continuously withry nitrogen gas at room temperature to minimize IR absorptiony atmospheric water vapor and carbon dioxide.

D) and (c) singly hydrogen bonded dimer (SHBD) of 2-pyrrolidinone calculated at

2.2. Computational

Geometry optimizations and normal mode calculations of themonomer and two hydrogen bonded dimers were performed bydensity functional theory (DFT) method using B3PW91 functionaland 6-311++G** basis set. Potential energy distributions (PED) ofthe normal modes were calculated using the method suggested byPulay et al. [12]. All quantum chemistry calculations presented herewere performed using GAMESS(US) program package [13].

3. Results and discussion

3.1. Molecular geometry

To our knowledge, no microwave spectroscopy data on geomet-ric parameters of 2PYD is available. However, rotational constants

128 P. Pandey et al. / Vibrational Spectroscopy 55 (2011) 126–131

Table 1Geometrical parameters of monomer and two dimers of 2-pyrrolidinone for opti-mization at DFT/B3PW91/6-311++G** level of calculation.

Geometrical parameters Monomer DHBD SHBD

C1–O3 (Å) 1.213 1.230 1.222C1–N6 (Å) 1.367 1.346 1.355N6–H13 (Å) 1.008 1.029 1.023C14–O16 (Å) – 1.230 1.224C14–N19 (Å) – 1.346 1.357N19–H26 (Å) – 1.029 1.008O16–H13 (Å) – 1.843 1.909O3–C1–N6 (◦) 126.002 126.309 126.423C1–N6–H13 (◦) 119.962 121.071 121.264O16–C14–N19 (◦) – 126.311 125.023C14–N19–H26 (◦) – 121.071 120.465O3–C1–N6–H13 (◦) 8.254 5.765 7.669O16–C14–N19–H26 (◦) – 5.843 6.037C1–C2–C4–C5 (◦) 25.118 24.611 24.511C14–C15–C17–C18 (◦) – 24.597 24.683C2–C4–C5–N6 (◦) −27.444 −26.204 −26.493

◦

obso[cbopSl

bpfagipeeoNao(iea

3

mp1ibsofitt

Fig. 2. Mid-FTIR spectrum of 2-pyrrolidinone in N2 matrix at 8 K. The upperpanel of the inset displays amide-I spectral profile in the matrix before and after

C15–C17–C18–N19 ( ) – −26.220 −26.057C5–N6–C1–C2 (◦) −5.288 −3.358 −4.188C18–N19–C14–C15 (◦) – −3.415 −3.402

f its nearest six-member analogue, 2-piperidinone, was measuredy Kuze et al. using microwave spectroscopy [14], and we havehown recently that excellent predictions for these parameters arebtained by calculation at DFT/B3PW91/6-311++G** level of theory15]. In the present study, we have used the same method for cal-ulation of the geometry of the monomer as well as two hydrogenonded dimers. We show below that the simulated infrared spectraf these species obtained by this method, particularly in the finger-rint region, display excellent agreement with the measured ones.uch agreement establishes the reliability of the method, particu-arly with respect to the goal of the presented study.

The optimized geometries of 2PYD monomer and two hydrogenonded dimers computed at DFT/B3PW91/6-311++G** level areresented in Fig. 1. The monomer is optimized in a half-chair con-ormation (part a) and it is seen that the same shape is also retainedfter dimerization. However, significant changes of some importanteometric parameters are noticed for several internal coordinatesnvolving the hydrogen bonded interface, but the remote skeletalart shows almost no change. In Table 1 some key geometric param-ters are presented. In DHBD (Fig. 1b), the C O and N–H bonds arelongated from monomer by 0.017 and 0.021 A, respectively. On thether hand, in case of SHBD (Fig. 1c), the C O of one moiety and–H of the other remain free, and the two hydrogen bonded groupsre elongated by 0.011 and 0.015 A, respectively. The bond lengthf the free N–H of SHBD is predicted equal to that of monomer1.008 A). The H-bond length in DHBD is 0.066 A shorter than thatn SHBD. We show below that such changes in geometrical param-ters of the two dimers are profoundly reflected in the measureds well as predicted spectra.

.2. FTIR spectra

The matrix isolation–FTIR spectrum of 2PYD measured in theid-IR spectral range (3600–450 cm−1) in N2 matrix at 8 K tem-

erature is presented in Fig. 2. The most intense band around700 cm−1 can easily be associated with the amide-I mode (primar-

ly �C O) of the molecule and the other band near 3470 cm−1 cane corresponded to amide-A mode, which primarily involve N–Htretching. These two modes are of central importance in the study

f the structural aspects of peptide backbones (–C(O)–N(H)–). Thengerprint region (1550–450 cm−1) is very rich in vibrational struc-ures and the bands correspond to various ring skeleton modes ofhe molecule.

annealing, and the lower panel displays predicted transitions of monomer, singlyhydrogen bonded dimer (SHBD) and doubly hydrogen bonded dimer (DHBD) atDFT/B3PW91/6-311++G** level of theory.

3.2.1. Amide-I bandThe amide-I band profile of 2PYD in N2 matrix is shown in the

inset of Fig. 2. The upper panel of the inset displays a section of thespectra around this band recorded in N2 matrix before and afterannealing (dashed and solid lines, respectively), and the theoret-ically predicted transitions of modes for the monomer and twodimers are shown in the lower panel. In the experimental spec-tra (upper panel), two bands around 1699 and 1712 cm−1 increasein intensity upon annealing the matrix, and this happens at expenseof the two other intense bands at 1724 and 1734 cm−1. Since thefraction of molecules existing as hydrogen bonded complexes isexpected to be smaller in the matrix before annealing, comparedto that in the annealed one, we assign the latter two bands tomonomer. The most intense one at 1734 cm−1 is assigned to amide-I fundamental, and somewhat weaker one at 1724 cm−1 to a Fermiresonance combination, where the most likely components are fun-damentals of modes 31 and 16 (see Table 2), respectively, and whichappear in the spectra at 470 and 1256 cm−1. The calculated har-monic frequencies of those two modes are 462 and 1252 cm−1,respectively. We assign the two other bands at 1699 and 1712 cm−1

to amide-I modes of DHBD and SHBD, respectively. The frequenciesof the theoretically predicted fundamentals of amide-I modes ofmonomer and two dimers are scaled by a factor of 0.958 to compen-sate for the high anharmonicity of the amide-I modes and displayedas stick plot in the lower panel of the inset. The agreement betweenthe measured and computed spectra appears to be very good.

We reiterate that the SHBD, which has been identified in thematrix observing the shifted �C O transition at 1712 cm−1, andwhose corresponding transition is absent in the FTIR spectrum ofCCl4 solution, is a transient species in the latter. The spectra of0.017 M CCl4 solution, recorded at a series of temperatures withinthe range 30–45 ◦C, are shown in part a, Fig. 3. A systematic changein spectral profile due to interconversion between monomer andhydrogen bonded complex of 2PYD is revealed here. The mostimportant feature of the changes depicted is that each of these spec-tra passes through a common point, the so-called ‘isosbestic point’

at 1709 cm−1. The appearance of such isosbestic point indicatesthat the equilibrium is established only between two components,and the most natural choices for these two components are themonomer and DHBD. Such correlation between an isosbestic point

P. Pandey et al. / Vibrational Spectroscopy 55 (2011) 126–131 129

Table 2Assignments of the monomer bands in the FTIR spectrum of 2-pyrrolidinone. Calculated vibrational frequencies (at B3PW91/6-311++G** level of theory) of modes 1 and 8are scaled by a factor of 0.958. For other modes a scaling factor of 0.985 is used.

Mode no.a Mode description in terms of PEDb Calculated frequency (cm−1) Observed band (cm−1)

1 �H13–N6 (100) 3476 34732 �H7–C2 (75), �H8–C2 (15) 30893 �H9–C4 (79), �H7–C2 (11) 30764 �H11–C5 (84) 30325 �H8–C2 (83), �H7–C2 (14) 30246 �H10–C4 (82), �H9–C4 (9) 30027 �H12–C5 (90) 29748 �O3–C1 (80) 1733 17349 ıH12–C5–H11 (90) 1510 149710 ıH10–C4–H9 (84) 1473 146411 ıH8–C2–H7 (88) 144112 �N6–C1 (23), ıH13–N6–C1 (23), �H12–C5–H11–N6 (20) 1424 142213 ıH13–N6–C1 (27), �H12–C5–H11–N6 (15), ıH9–C4–C5 (12) 1348 134314 �H10–C4–H9–C5 (50), �H12–C5–H11–N6 (19) 1311 131015 �H8–C2–H7–C1 (44) 1283 128616 �N6–C1 (19), �H12–C5–H11–N6 (17), ıH7–C2–C1 (14), ıH13–N6–C1 (11), ıH9–C4–C5 (10) 1252 125617 ıH11–C5–N6 (29), �H11–C5–N6–C4 (12), �H7–C2–C1–C4 (11) 1228 122618 ıH9–C4–C5 (31), �H9–C4–C5–C2 (11), �C4–C2 (10), ıH7–C2–C1 (9) 1192 119819 ıH11–C5–N6 (26), ıH7–C2–C1 (14), �H12–C5–H11–N6 (11), �H8–C2–H7–C1 (10), �H10–C4–H9–C5 (9) 1167 117020 ıN6–C1–C5 (18), �C4–C2 (13), �H11–C5–N6–C4 (12), �C5–C4–C2–C1 (11), ıH7–C2–C1 (10) 1078 107921 ıN6–C1–C5 (16), ıH7–C2–C1 (13), �H9–C4–C5–C2 (13), �H8–C2–H7–C1 (10), ıC5–C4–C2 (9) 1066 106322 ıC4–C2–C1 (18), �C4–C2 (13), �C5–C4 (12), ıC5–C4–C2 (10) 994 99123 �C5–C4 (40), �C4–C2 (27) 91924 �H11–C5–N6–C4 (34), ıH9–C4–C5 (16), �H7–C2–C1–C4 (11) 89325 �C2–C1 (22), �C5–C4 (16), �N6–C1 (15), �H7–C2–C1–C4 (14), ıC5–C4–C2 (13) 880 88326 �C2–C1 (27), �H9–C4–C5–C2 (21), �H7–C2–C1–C4 (14), �C4–C2–C1–O3 (11) 804 81127 ıC5–C4–C2 (24), ıC4–C2–C1 (18), ıN6–C1–C5 (17) 685 68828 �C4–C2–C1–O3 (30), ıC4–C2–C1 (17) 628 63129 ıC4–C2–C1 (26), �C4–C2–C1–O3 (18), �H13–N6–C1–C5 (15), �H7–C2–C1–C4 (10) 557 56430 �H13–N6–C1–C5 (38), �C4–C2–C1–O3 (26), �N6–C1–C5–O3 (15) 473 49131 ıO3–C1–C2 (55), �H13–N6–C1–C5 (11), �N6–C1–C5–O3 (9) 462 47032 �H13–N6–C1–C5 (27), �N6–C1–C5–O3 (13) 19733 �N6–C1–C5–O3 (84) 145

s than

abdrcoitDtmbuttrrothfgtt

3

aat

a Standard Mulliken scheme is used for mode numbering.b Values within parentheses indicate percentage contributions; contributions les

nd two different forms of molecular species in equilibrium haseen observed in many cases [16–20]. Lower panel (part b) of Fig. 3isplays the curve fitting of amide-I band profile for solution atoom temperature (22 ◦C). It shows that the observed band profilean nearly be reproduced by considering three component bandsf Voigt type profile. The bands of maxima at 1714 and 1729 cm−1

ncrease in intensity with increase in temperature at the expense ofhe third one at 1699 cm−1. We assign the latter to amide-I band ofHBD, and the former two bands to monomer. Between these two,

he more intense one at 1714 cm−1 can be assigned to amide-I ofonomer, and the weaker one at 1729 cm−1 to a Fermi resonance

and. Thus, according to our assignment, the amide-I band of 2PYDndergoes a large, 20 cm−1, red shifting in CCl4 solution comparedo its frequency in N2 matrix. The Fermi resonance band is assignedo the same combination. However, its frequency in CCl4 expe-iences an apparent blue shifting. This happens because of largeed-shifting of amide-I band, and the shifted frequency now turnsut to be smaller than the combination. Fermi resonance interac-ion now shifts the combination to opposite direction. On the otherand, the amide-I band of DHBD does not display any solvation shift

rom N2 matrix to CCl4 solution. This is not surprising as the amideroup of one molecule is already solvated by hydrogen bond withhe other molecule. Such effects were also observed in the case ofhe dimer of �-valerolactam [15].

.2.2. Amide-A bandAmide-A bands of 2PYD and its dimers are shown in Fig. 4. Parts

and b represent the spectra recorded in N2 matrix, before and afternnealing, respectively, and the spectrum of 0.017 M CCl4 solu-ion at room temperature is displayed at the bottom panel (part

9% have not been presented.

c), and it looks similar to the one reported earlier by Walmsley[3]. The sharp band at 3455 cm−1 in the spectrum of panel c cor-responds to �N–H of 2PYD monomer. In N2 matrix the same bandis shifted to higher frequency at 3473 cm−1. Two bands at ∼3231and ∼3194 cm−1 increase in intensity after annealing the matrix,but it appears that the latter increases more than the former. Weassign the former band to the stretching of hydrogen bonded N–H(�N–H

HB) of the SHBD and the latter to the antisymmetric �N–H ofthe DHBD. There is also a distinct band at 3408 cm−1 in matrix iso-lation spectrum, which rises in intensity in the annealed matrix. Asthe band is absent in CCl4 solution, it must be associated with theSHBD which is a transient species in CCl4 solution at room temper-ature. We assign this peak to the overtone of amide-I (1712 cm−1)of the SHBD.

The relative spectral positions of amide-A frequencies ofmonomer as well as two hydrogen bonded dimers are satisfac-torily consistent with our theoretical predictions. The N–H bondlength of monomer and the bond length of the free N–H of SHBD arepredicted to be the same (see Table 1). The H-bond induced elon-gation of N–H bond in SHBD is 0.015 A but in DHBD the N–H bondelongates by 0.021 A. Furthermore, the calculated H-bond length inDHBD (1.843 A) is considerably shorter than that in SHBD (1.909 A).In Table 3 we have summarized the predicted and observed amide-A frequencies of all three molecular species. The table shows thatthe agreement between calculated and observed frequencies is

satisfactory in general. Based on the prediction that the stretch-ing of free N–H (�N–H

free) of SHBD undergoes a very small shift(��N–H = 9 cm−1) from the �N–H of monomer, we suggest that the�N–H

free of SHBD and the �N–H of monomer are superposed at nearlysame frequency. The bond length changes and �N–H frequency

130 P. Pandey et al. / Vibrational Spectroscopy 55 (2011) 126–131

Table 3A comparison between the calculated (at B3PW91/6-311++G** level of theory) and observed frequencies of amide-A modes of 2-pyrrolidinone monomer and its hydrogenbonded dimers.

�N–H (molecular species) Calculated frequency scaled by 0.958 (cm−1) Observed band position (cm−1)

N2 matrix CCl4 solution

Monomer 3476DHBD (antisymmetric �N–H) 3178SHBD (free N–H) 3485SHBD (H-bonded N–H) 3280

Fig. 3. Spectral evolution of amide-I band of 2-pyrrolidinone with variation of tem-perature in 0.017 M CCl4 solution (part a) and curve fitting of the same band at 22 ◦Cby three voigt profiles (shown by blue curves) in part b. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthe article.)

Fig. 4. A comparison of amide-A bands of 2-pyrrolidinone and its dimers in N2

matrix at 8 K before (a) and after (b) annealing, and 0.017 M CCl4 solution at 22 ◦C(c).

3473 3455∼3194 ∼3200∼3473 –∼3231 –

shifting in DHBD are clearly predicted to be enhanced owing tocooperative interactions between the two H-bonds. In the matrixisolation spectrum, the amide-A band experiences a red shifting of∼240 cm−1 in SHBD, but in DHBD the extent of shift is more than270 cm−1.

3.2.3. Fingerprint bandsFig. 5a displays the 1550–450 cm−1 segment (fingerprint region)

of the matrix isolation–FTIR spectrum of 2PYD measured in 8 Knitrogen matrix. The predicted gas phase spectrum in this spec-tral range at DFT/B3PW91/6-311++G** level of calculation has beenshown in the lower panel (Fig. 5b). Lorentzian band profile ofFWHM 2 cm−1 has been used to display the calculated spectrum,and to compensate the anharmonic effects a scale factor of 0.985has been used. It is evident that the band positions as well as rel-ative intensities of most of the infrared active bands are predictedquite satisfactorily by theoretical calculation. Definition of eachmode in terms of potential energy distribution (PED) is presentedin Table 2. It is noteworthy that two bands centered around 500 and564 cm−1appear distinctly broader than the rest of the fingerprintbands. PED suggests that out-of-plane NH bending (�H13–N6–C1–C5)has major contribution to those modes. Such vibrational modesare known as ‘amide-V’ modes. There are at least two extra bandsobserved in the fingerprint region, one at 1170 and the other at

1276 cm−1, which are not predicted by harmonic calculations. Thus,we infer that these two bands are some combinations and/or over-tones of lower frequency modes. The 1170 cm−1 band has beenassigned as combination of amide-V fundamental (∼500 cm−1)

100011001200130014001500 500600700800900

688

Wavenumber (cm-1

)

Inte

nsi

ty

b Predicted

500

1510

1473

1424

1348 1

311

1283

1252

1228

1192

1167

1078

1066

994

880

804

685

628

557

473 462

1497

1464

1422

1343

1310

1198

1286

1276

1256

1226

1170

1160

1079

8839

91

1063

811

a Experimental

631

564

470

Fig. 5. The fingerprint region of the FTIR spectrum of 2-pyrrolidinone in N2 matrixat 8 K is presented in an expanded scale in upper panel. The simulated spectrum formonomer of 2-pyrrolidinone calculated at DFT/B3PW91/6-311++G** level for thesame spectral range is presented in the lower panel.

P. Pandey et al. / Vibrational Spect

100011001200130014001500

Inte

nsi

ty

Wavenumber (cm-1

)

Theory (DHBD)

Theory (SHBD)

Theory (monomer)

Expt. (annealed matrix)

Expt. (matrix before

annealing)

Fig. 6. The spectral manifestations of complex formation in the fingerprint regionoccurring on annealing the matrix are displayed in the top two panels. Theoreticallyscb

wa1taf(

(sOistst

[[[[

[

[

imulated spectra of monomer, SHBD and DHBD at DFT/B3PW91/6-311++G** level ofalculation are also presented. In the annealed matrix spectrum the newly developedands are indicated by arrows.

ith 688 cm−1 band, the 1276 cm−1 band as a combination of 470nd 811 cm−1 bands. Two most intense bands in this region at256 and 1422 cm−1 are assigned to modes 16 and 12, respec-ively, and PED in Table 2 shows that they have somewhat similartomic displacement pattern, and both have major contributionsrom C–N stretch, in-plane NH bending and wagging of CH2 group�H12–C5–H11–N6).

Fig. 6 displays the effect of annealing on the fingerprint region1550–1000 cm−1) of the matrix isolation–FTIR spectrum, and theimulated spectra of monomer, SHBD and DHBD are also displayed.n annealing, a number of new bands appear weakly and those are

ndicated by arrows. Comparison of the observed and simulatedpectra reveals that those new bands can be corresponded to thewo dimers. Those dimer features do not appear in the spectrumhown at the top of the figure because of their low population inhe matrix before annealing.

[[[[[

roscopy 55 (2011) 126–131 131

4. Conclusion

2-Pyrrolidinone and its two hydrogen bonded dimers (DHBDand SHBD) have been identified by FTIR spectroscopy in cold N2matrix. Comparison of the spectra with those recorded in CCl4solution reveals that the SHBD is transient in solution phase atroom temperature. Red shifting of amide-I and amide-A bandswith formation of two successive H-bonds reveals cooperativestrengthening of H-bonds in parallel to the predicted H-bond con-traction in DHBD. The effect is more pronounced in �N–H (amide-A)region as the H-bond induced distortion of N–H bond is compar-atively large than that of C O bond. Theoretical predictions atDFT/B3PW91/6-311++G** level are in good agreement to the exper-imental observations.

Acknowledgments

The authors gratefully acknowledge the financial supportreceived from CSIR and Ramanna Fellowship grant of DST, Govt.of India, to carry out the research. PP, BB and AKS thank DRDO,CSIR and IACS, respectively, for Senior Research Fellowships.

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