Monothiodibenzoylmethane: Structural and vibrational assignments

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Vibrational Spectroscopy 43 (2007) 53–63

Monothiodibenzoylmethane: Structural and vibrational assignments

Bjarke K.V. Hansen a, Alexander Gorski b, Yevgen Posokhov b, Fritz Duus a,Poul Erik Hansen a, Jacek Waluk b, Jens Spanget-Larsen a,*

a Department of Life Sciences and Chemistry, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmarkb Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01-224 Warsaw, Poland

Received 27 April 2006; received in revised form 12 June 2006; accepted 13 June 2006

Available online 7 August 2006

Abstract

The vibrational structure of the title compound (1,3-diphenyl-3-thioxopropane-1-one, TDBM) was studied by a variety of experimental and

theoretical methods. The stable ground state configuration of TDBM was investigated by infrared (IR) absorption measurements in different media,

by linear dichroism (LD) polarization spectroscopy of samples partially aligned in a stretched polymer matrix, and by Raman spectroscopy.

The investigation of the metastable photoproduct of TDBM was based on the previously published spectrum of the product trapped in argon matrix

[Y. Posokhov, A. Gorski, J. Spanget-Larsen, F. Duus, P.E. Hansen, J. Waluk, Chem. Phys. Lett. 350 (2001) 502]. The observed vibrational spectra

were compared with theoretical transitions obtained with B3LYP/cc-pVTZ density functional theory (DFT). The results leave no doubt that the

stable ground state configuration of TDBM corresponds to the intramolecularly hydrogen bonded-enol form (e-CCC), and that the photoproduct

corresponds to an ‘‘open’’, non-chelated enethiol form (t-TCC), thereby supporting the previous conclusions by Posokhov et al. No obvious

indications of the contribution of other forms to the observed spectra could be found.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Thioketones; b-Thioxoketones; Infrared spectroscopy; Linear dichroism; Raman spectroscopy; Intramolecular hydrogen bonding; Density functional

theory (DFT)

1. Introduction

b-Thioxoketones have been studied for decades, with

particular attention paid to their chelating properties [1,2]

and to their tautomeric and photochromic behaviour [3–27]. Of

particular interest is their ability to undergo photoconversion at

low temperatures on irradiation with UV–vis light [10], but the

mechanism of the reaction and the structure of the metastable

photoproduct has been a subject of debate [10,13,14,23,25–27].

The b-thioxoketone photoreactivity was first investigated by

Carlsen and Duus [10], who concluded that the photoprocess

was best interpreted as the transformation of an initial (Z)-enol

tautomeric form exhibiting a strong intramolecular hydrogen

bond into a (Z)-enethiolic counterpart, e.g., e-CCC! t-CCC

(Scheme 1; same nomenclature as in Ref. [26]). In a subsequent

study of thioacetylacetone in an argon matrix, Gebicki and

Krantz [13,14] interpreted the photoprocess as transformation

* Corresponding author. Tel.: +45 46742710; fax: +45 46743011.

E-mail address: [email protected] (J. Spanget-Larsen).

0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.vibspec.2006.06.015

of an initial (Z)-enethiol form (corresponding to t-CCC) to

another (Z)-enethiol form obtained by rotation around the

central C–C bond (approximately corresponding to t-TCT).

These authors excluded the presence of an initial (Z)-enol form

(e.g., e-CCC). On the other hand, X-ray and neutron diffraction

investigations [4,5] of monothiodibenzoylmethane (TDBM)

indicated the predominance of the chelated enolic form e-CCC,

thus questioning the interpretation by Gebicki and Krantz

[13,14].

Posokhov et al. [25] recently investigated the photoreactivity

of TDBM by infrared (IR) and ultraviolet–visible (UV–vis)

spectroscopy and by density functional theory (DFT) calcula-

tions. On the basis of a comparison between the observed IR

spectra and those predicted for a variety of structures, it was

concluded that the stable ground state structure of TDBM is the

chelated (Z)-enol form e-CCC, consistent with the X-ray and

neutron diffraction results [4,5], and that the photoproduct

corresponds to a non-chelated–SH exo-rotamer of the (Z)-

enethiolic tautomeric form, i.e., t-TCC. The observed activation

barrier for the reverse ground state process that brings the

metastable photoproduct back to the initial stable structure was

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http://dx.doi.org/10.1016/j.vibspec.2006.06.015

B.K.V. Hansen et al. / Vibrational Spectroscopy 43 (2007) 53–6354

Scheme 1.

found to be equal to 6.9 � 0.4 kcal/mol [25,27]. This value is

consistent with the calculated potential energy barrier for

rotation of the –SH group from the exo-position in t-TCC to the

endo-position in t-CCC (B3LYP/cc-pVDZ: 9.45 kcal/mol)

[28]. However, no contributions from the t-CCC form to the

observed optical spectra could be identified, suggesting that this

configuration of TDBM is rapidly transformed to the stable (Z)-

enol form e-CCC [25,28].

In the present investigation, we substantiate the results of

Posokhov et al. [25] by detailed analyses of the molecular

vibrations of the e-CCC and t-TCC forms of TDBM,

corresponding to the equilibrium configurations of the stable

ground state species and the metastable photoproduct. For

the stable e-CCC species, we present mid-IR and far-IR

spectra measured in a variety of media, as well as a Raman

powder spectrum. The analysis is supported by linear

dichroism (LD) [29–32] spectroscopy on molecular samples

partially aligned in a stretched polyethylene (PE) matrix.

These data provide information on the molecular transition

moment directions and help to resolve otherwise hidden,

differently polarized transitions. The investigation of the

metastable t-TCC form is based on the previously published

IR spectrum of the photoproduct trapped in argon matrix at

20 K [25]. The measured transitions are assigned to

theoretical transitions calculated with DFT procedures.

The results of the present study fully support the previous

conclusions of Posokhov et al. [25], and demonstrate

that DFT is a very useful calculational tool in the

investigation of the molecular and vibrational structures of

b-thioxoketones.

2. Experimental and computational details

TDBM was prepared and purified as previously described

[6,8]. IR absorption spectra of TDBM and its photoproduct in

argon matrices at 20 K have been previously published [25].

Additional IR spectra of TDBM were measured at room

temperature on a Perkin-Elmer Spectrum 2000 FTIR spectro-

photometer equipped with FR-DTGS mid-IR (KBr) or FR-

DTGS far-IR (Poly) detectors. A solid state KBr (Merck

UVASOL) tablet spectrum of TDBM was recorded with a

spectral resolution of 2 cm�1 and 500 scans. Liquid solution

spectra were measured in CCl4 and CS2 solvents (Merck

UVASOL) with a spectral resolution of 2 cm�1 and 100 scans.

A far-IR spectrum was recorded using a PE tablet (Merck,

UVASOL) solid state sample with a resolution of 1 cm�1 and

1000 scans (250 1–4–1 cycles, dry nitrogen flushed). In

addition, a number of IR experiments were performed by using

a wider variety of solvents and with sample temperatures

ranging from 20 to 413 K.

Stretched low-density PE samples for LD spectroscopy [29–

33] were prepared from ca. 2 mm thick PE material. TDBM

was introduced into the unstretched polymer by sublimation at

50 8C for 2 weeks. Excess TDBM was removed from the

surface of the sample with methanol (Merck, UVASOL) and the

sample was uniaxially stretched by ca. 500%. A similar sheet

without TDBM was produced for use as a reference. The LD

spectra were recorded with a rotatable KRS5 aluminum grid in

the sample beam (resolution of 2 cm�1 and 500 scans). Two

linearly independent absorbance curves were measured, one

with the electric vector of the sample beam parallel to the

stretching direction (U), and one with the electric vector

perpendicular to it; in both cases, the beam was perpendicular

to the surface of the PE sample. The resulting baseline-

corrected absorbance curves are denoted by EUðnÞ and EVðnÞ.Because of strong PE baseline absorption, the regions 700–730,

1350–1380, 1430–1480 and around 3000 cm�1 could not be

investigated with the PE technique.

A solid state Raman spectrum of TDBM was measured at

room temperature on a Bruker-IFS66 interferometer equipped

with a FRA 106 FT-Raman module and a liquid nitrogen cooled

Ge-detector. The Raman signal was excited with a Nd/YAG

laser at 1064 nm (output 25 mW, resolution of 2 cm�1 and 500

scans).

Quantum chemical calculations were performed with the

GAUSSIAN 03 suite of programs [34] by using the B3LYP

density functional [35,36] and the cc-pVTZ basis set [37]. The

fundamental vibrational transitions were computed within the

harmonic approximation. Complete listings of the predicted

transitions for the e-CCC and t-TCC configurations are shown

in Tables S1 and S2 in Supplementary data. Selected results are

given in Tables 2 and 3; in these tables, the theoretical

wavenumbers were multiplied by the scaling factor 0.977.

3. Results and discussion

3.1. Stable ground state species (e-CCC)

The experimental spectra are shown in Figs. 1–4 and

measured and computed transitions are listed in Tables 1 and 2.

An overview of recorded isotropic spectra is given in Fig. 1 and

Table 1. The comparison indicates that the spectra show only

the usual medium-induced shifts. Moreover, the relative

intensities of the observed transitions were insignificantly

affected by variation of the temperature within the range 20–

413 K (not shown). The additional information that can be

extracted from the LD curves EUðnÞ and EVðnÞ in Fig. 3 are the

orientation factors Ki for the transition moments of the observed

transitions i [29–32]:

Ki ¼ hcos2 ðMi;UÞi:

Here (Mi, U) is the angle between the transition moment

vector Mi of transition i, and the uniaxial stretching direction U

of the polymer sample. The pointed brackets indicate averaging

over all molecules in the light path. The Ki values for TDBM

were determined by the TEM procedure [29–33], which

involves formation of linear combinations of EUðnÞ and EVðnÞ,

B.K.V. Hansen et al. / Vibrational Spectroscopy 43 (2007) 53–63 55

Fig. 1. (Top) IR absorption spectra of TDBM measured at 20 K in argon matrix

[25], and at room temperature in stretched polyethylene (Eiso = (EU + 2EV)/3),

in liquid CCl4 (>1400 cm�1) and CS2 (<1400 cm�1), and in solid state KBr

tablet. The curves are successively displaced by 1.5 absorbance units. (Bottom)

Vibrational absorption spectrum of the e-CCC configuration of TDBM com-

puted with B3LYP/cc-pVTZ; wavenumbers are scaled by a factor of 0.977 and a

Lorentz line-shape with HWHM = 5 cm�1 is assumed.

Fig. 2. (Top) Far-IR absorbance spectrum of TDBM measured at room tem-

perature in polyethylene tablet. (Bottom) Vibrational absorption spectrum of the

e-CCC configuration of TDBM computed with B3LYP/cc-pVTZ, scaling and

line-shape as in Fig. 1.

Fig. 3. (Top) IR linear dichroism (LD) absorbance curves EUðnÞ (full line) and

EV ðnÞ (dashed line) measured at room temperature for TDBM partially aligned

in stretched polyethylene. (Bottom) Family of reduced absorbance curves rKðnÞwith K ranging from 0.0 to 1.0 in steps of 0.1. Estimated orientation factors K

are indicated for a number of peaks (see text).

Fig. 4. (Top) Raman spectrum of TDBM powder measured at room tem-

perature. (Bottom) Vibrational Raman spectrum of the e-CCC configuration

of TDBM computed with B3LYP/cc-pVTZ, scaling and line-shape as in

Fig. 1.


Table 1

TDBM: observed vibrational wavenumbers (cm�1) and intensities (vw, very weak; w, weak; m, medium; s, strong)

IR Raman powderb

Ar matrixa KBrb,c PEb,d CCl4b,e CS2

b,e

3079 (w) 3085 (w) 3081 (w) 3076 (m)

3051 (w) 3065 (w) 3062 (w) 3057 (w)

3031 (vw) 3036 (w) 3036 (w) 3033 (vw)

3022 (w) 3026 (vw) 3024 (vw)

2998 (vw) 2995 (vw)

2972 (vw)

1602 (m) 1597 (m) 1600 (w) 1601 (w) 1599 (w)

1591 (m) 1588 (m) 1588 (m) 1588 (w) 1587 (m) 1594 (vs)

1561 (vs) 1555 (vs) 1555 (vs) 1555 (vs) 1548 (vs)

1494 (m) 1491 (m) 1492 (m) 1492 (m) 1489 (w)

1464 (s) 1461 (m) 1461 (m) 1458 (w)

1448 (w) 1447 (w)

1421 (w) 1421 (w) 1417 (w) 1418 (w) 1416 (w) 1418 (vw)

1399 (vw)

1365 (w) 1363 (vw) 1356 (w)

1340 (w) 1338 (vw)

1313 (w) 1316 (w) 1310 (w) 1312 (w) 1310 (w)

1290 (w) 1293 (m) 1294 (w) 1289 (w) 1289 (w)

1294 (m)

1273 (m) 1273 (m) 1267 (w) 1269 (w) 1268 (m) 1275 (s)

1255 (w)

1246 (m) 1248 (m) 1247 (m) 1247 (m) 1246 (m) 1243 (s)

1238 (m) 1239 (m) 1236 (w) 1238 (w) 1236 (m)

1212 (w) 1212 (w) 1209 (w) 1209 (w) 1211 (m)

1202 (m) 1202 (w) 1200 (w) 1203 (w) 1199 (m)

1188 (w) 1185 (w) 1185 (w) 1185 (w) 1183 (w) 1186 (m)

1173 (vw)

1159 (w) 1160 (w) 1158 (w) 1159 (vw) 1158 (w) 1162 (w)

1112 (vw) 1109 (vw)

1099 (m) 1099 (w) 1098 (w) 1100 (w) 1099 (w) 1096 (vw)

1080 (w) 1080 (w) 1075 (vw)

1069 (m) 1069 (w) 1070 (w) 1068 (w) 1067 (w) 1068 (vw)

1047 (vw) 1041 (vw)

1033 (w) 1027 (w) 1031 (w) 1031 (w) 1030 (w) 1029 (w)

1001 (w) 999 (w) 1000 (w) 1001 (vw) 1000 (vw) 998 (s)

990 (vw)

969 (w) 967 (vw) 966 (vw) 966 (vw) 967 (vw) 968 (vw)

952 (w) 959 (vw)

946 (vw) 948 (w) 951 (w) 950 (w) 945 (vw)

929 (w) 928 (vw)

922 (w) 923 (vw) 922 (w) 925 (w) 919 (w)

851 (w) 846 (vw)

839 (w) 837 (w) 836 (w) 838 (w) 836 (w) 837 (m)

825 (w) 820 (m) 817 (w) 819 (w) 818 (vw)

780 (w) 779 (w) 776 (w) 776 (w) 782 (vw)

764 (m) 763 (m) 760 (m) 761 (m)

738 (w) 733 (w) 735 (w) 735 (w) 732 (w)

695 (m) 696 (w)

686 (w) 690 (m) 687 (m) 692 (m) 690 (m)

679 (w)

667 (w) 667 (w) 666 (w) 667 (w) 667 (w) 665 (w)

628 (w) 626 (vw) 624 (vw) 627 (vw) 626 (vw) 625 (vw)

618 (vw) 617 (vw) 618 (vw) 615 (vw)

612 (w) 614 (vw) 613 (vw) 613 (vw)

571 (w) 568 (w) 569 (w) 570 (w) 569 (w) 567 (vw)

480 (w) 482 (w) 477 (w) 478 (w)

472 (w) 466 (w) 471 (w)

434 (vw)

407 (vw) 404 (vw) 408 (w)

373 (w) 397 (w)

371 (vw)

332 (w)


Table 1 (Continued )

IR Raman powderb

Ar matrixa KBrb,c PEb,d CCl4b,e CS2

b,e

269 (vw) 267 (vw)

244 (vw) 245 (m)

202 (w)

173 (w)

a 20 K.b Room temperature.c KBr tablet (>500 cm�1) and PE tablet (<500 cm�1).d Stretched polyethylene, Eiso = EU + 2EV.e Liquid solution.

{Table 2

TDBM: assignment of observed IR and Raman bands to fundamental transitions computed for the e-CCC configuration

KBra PEb Ramanc B3LYP/cc-pVTZ

nd Ie nd Kf nd Ag nd,h Ie jaji Ag App. mode descriptionj

n1 3146 6 638 32 CH s

n2 3140 6 198 47 CH s

3079 11 n3 3128 10 248 80 CH s

3051 13 n4 3126 9 838 57 CH s

3031 4 n5 3120 25 758 112 CH s

3022 8 n6 3117 44 98 99 CH s

2998 4 n7 3112 34 208 59 CH s

2972 2 n8 3107 13 748 72 CH s

n9 3102 11 838 70 CH s

n10 3097 1 48 24 CH s

n11 3093 1 188 21 CH s

1597 58 1600 0.49 1594 371 n13 1606 13 378 482 Skel def (B)

n14 1600 5 698 230 Skel def (A)

1588 103 1588 0.51 n15 1588 166 68 77 OH b, C C s, skel def (B)

1555 531 1555 0.51 1548 279 n17 1559 1264 38 386 C C s, OH b

1491 59 1492 0.52 1489 9 n18 1495 89 58 23 Skel def (B), OH b

1461 108 1458 8 n20 1464 159 538 33 C–O s, C–C s

1447 14 n22 1424 92 378 27 OH b, CH b

1421 18 1417 0.33 1418 1 n23 1399 151 248 42 OH b, C–O s, C–C s

1399 2

1363 3 1356 6

1340 7 1338 4

1316 23 1310 0.53 n24 1331 13 258 6 CH b, skel def (A)

1293 49 1294 0.41 n26 1299 108 78 6 Skel def, CH b (A)

1273 86 1267 0.41 1275 247 n28 1261 114 458 425 OH b, CC s, CH b

1255 40

1248 71 1247 0.50 1243 179 n29 1247 436 348 269 C2H b, OH b

1239 50 1235 0.42 n30 1203 48 888 76 CH b, skel def

1212 12 1209 0.41 1211

1202 19 1199

1185 33 1185 0.44 1186 65 n31 1184 7 378 41 CH b (B)

1173 4

1160 7 1159 0.40 1162 16

1112 1 1109 0

1099 12 1098 0.46 1096 1 n35 1089 28 408 1 Skel def, CH b, C–O s

1080 9

1069 33 1070 0.50 1068 2 n37 1068 48 108 2 C–O s, skel def

1041 0.20

1027 11 1031 0.55 1029 29 n38 1031 9 58 23 Skel def, CH b

1022 0.46 n39 1029 5 178 9 Skel def, CH b

999 9 1001 0.47 998 100 n40 998 7 68 12 Skel def (A,B)

n41 998 3 508 88 Skel def (A,B)

990 2

967 0 964 0.44 968 1

959 0

946 1 948 0.38 945 5 n46 951 45 198 5 Skel def, C S s

929 5 928 1


Table 2 (Continued )

KBra PEb Ramanc B3LYP/cc-pVTZ

nd Ie nd Kf nd Ag nd,h Ie jaji Ag App. mode descriptionj

923 4 922 0.17 n48 929 7 778 0 CH oop b (A)

846 1

837 22 836 0.25 837 36 n53 828 15 798 25 C2H oop b, OH tor

820 48 818 0.22 818 1 n52 838 25 648 10 C2H oop b, OH tor, C S s

779 15 777 0.24 782 2 n54 784 20 628 3 CH oop (A,B)

763 100 760 0.16 n55 768 100 828 0 CH oop (A)

733 18 732 27 n56 730 24 678 19 Skel def, C S s

696 30 n57 695 69 808 1 CH oop b (A)

690 62 690 0.26 n58 691 51 848 1 CH oop b (B)

679 15 687 0.16 n59 686 1 788 0 C2H oop b, OH tor

667 14 667k 0.3 665 11 n60 669 11 708 8 Skel def (A,B)

626 4 625 1

618 2 622 0.45 615 3 n62 620 1 178 4 Skel def (B)

614 2

568 14 569 0.53 567 3 n64 570 39 98 1 Skel def

480 14 478 19 n66 475 28 138 5 Skel def, CH oop b

472 10 469 0.32 471 15 n65 486 15 638 2 Skel def, CH oop b

434 1

407 3 402 408 9 n68 407 2 258 4 Skel def, CH oop b (A)

397

373 371 3 n70 366 6 608 1 C–O b, C S s

332 10 n71 329 1 258 2 C–O b, skel def

269 267

244 245 56 n74 225 1 728 7 Skel def

202 11 n75 199 1 228 2 C-O oop b, skel def

173 20 n76 146 1 308 2 Skel def

a KBr tablet (>500 cm�1) and PE tablet (<500 cm�1).b Stretched polyethylene.c Powder.d n, wavenumber (cm�1).e I, integrated IR intensity relative to I(n55) = 100.f K, orientation factor (see text).g A, Raman scattering activity relative to A(n40) + A(n41) = 100.h Scaling factor, 0.977.i a, transition moment angle with the molecular ‘‘long axis’’ x (Scheme 2).j s, stretch; b, bend; oop, out-of-plane; skel, skeletal; def, deformation; tor, torsion; A, thiobenzoyl ring; B, benzoyl ring (Scheme 2).k Overlapped by contribution from atmospheric CO2.

Scheme 2.

for example, the family of ‘‘reduced’’ absorbance curves rKðnÞ[33]:

rKðnÞ ¼ ð1� KÞEUðnÞ � 2KEVðnÞ:

A spectral feature due to transition i (a peak or a shoulder)

will disappear from the linear combination rKðnÞ for K = Ki,

and the value of Ki can thus be determined by visual inspection

[33]. An example of a family of rKðnÞ-curves for TDBM with

variation of K from 0.0 to 1.0 is shown in Fig. 3. The derived K-

values are given in Table 2.

While determination of the orientation factors Ki is usually

straightforward, it is much more difficult to derive the transition

moment directions within the molecular framework. The

observed K-values for TDBM range from 0.16 to 0.55, which

indicates a fairly efficient molecular alignment, consistent with

the elongated shape of the molecule. But the values do not show

the grouping characteristic for the presence of a molecular

symmetry element, such as a plane of symmetry [29–32]. This

can be explained by the twisting of the phenyl rings of TDBM

out of the plane of the central chelate ring moiety. The crystal

analysis by Richter et al. [5] obtained the dihedral angles 378for the thiobenzoyl ring (A in Scheme 2) and 138 for the other

ring (B in Scheme 2). The corresponding angles predicted with

B3LYP/cc-pVTZ are 378 and 188. This sterically induced

twisting reduces the molecular symmetry from Cs to C1. In the

oxygen analogue dibenzoylmethane-enol (DBM), the steric

demand is less significant and the molecule is nearly planar


[38], thereby greatly simplifying the analysis of the LD data for

this compound [39–41]. But in the case of TDBM, without the

help of molecular symmetry properties and without additional

information, the observed K-values allow only qualitative

conclusions on the molecular polarization directions.

The individual molecular transition moment directions may

be given in terms of their angles a, b, and g with a set of fixed

molecular axes x, y, and z, respectively. Our choice of axes for

TDBM is shown in Scheme 2: the axis x passes through carbon

centers 1 and 3, y is perpendicular to x and positioned in the

plane of the central, hydrogen-bonded moiety, and z is

perpendicular to x and y (not shown in Scheme 2). We shall

assume that the axis x coincides with the effective orientation

axis [29–32], the ‘‘long axis’’ of the molecule. Transitions with

small a-values, polarized essentially parallel to the orientation

axis x, should give rise to peaks with relatively large K-values.

And vice versa: those transitions with large a-values, i.e., those

with moments forming large angles with the orientation axis,

should correspond to peaks with relatively small K-values.

The assignments of the observed vibrational transitions to

the theoretical fundamentals calculated with B3LYP/cc-pVTZ

are listed in Table 2. The assignments are based on the

application of four criteria: wavenumber, IR intensity,

polarization data, and Raman activity. In those cases where

all (or several) criteria agree, the assignment seems relatively

secure. This is the case particularly for a number of strong

transitions. On the other hand, several problem cases remain, as

discussed in the following.

3.1.1. The region above 1700 cm�1

The most interesting fundamental in this region is the O–H

stretching mode, nOH. Within the harmonic approximation, this

mode is predicted by B3LYP/cc-pVTZ to give rise to a very

strong absorption band at 2786 cm�1 (n12 in Table S1 in

Supplementary data), but the observed spectra do not exhibit a

corresponding band in this spectral region, see Fig. 1. However, it

is well known that strong intramolecular hydrogen bonding leads

to large anharmonic effects, resulting in a shift of nOH towards

lower wavenumbers and possibly enhanced coupling with other

modes. The recent theoretical analysis by Szczepaniak et al. [42]

of the IR spectrum of picolinic acid N-oxide (PANO)

demonstrates how anharmonic effects associated with strong

intramolecular hydrogen bonding may lead to a drastic shift of

nOH towards lower wavenumbers and redistribution of the

associated IR intensity over several other modes. In the case of

DBM, inclusion of anharmonic effects by means of a second

order perturbation approximation (GAUSSIAN 03 option

‘‘freq = anharm’’ [34]) leads to a drastic lowering of the nOH

wavenumber: from 2904 to 2223 cm�1 with B3LYP/6-31G*

[41], and from 2663 to 1547 cm�1 with B3LYP/cc-pVDZ

[40,41]. A corresponding B3LYP/6-31G* calculation on TDBM

predicted an anharmonic shift from 2785 to 2185 cm�1 [41]. But

as indicated by the results for DBM, the predicted shift probably

depends critically on the choice of basis set. Unfortunately, we

have not succeeded in performing the anharmonic calculation on

TDBM with larger basis sets. Further discussion must await the

results of additional investigations.

3.1.2. The region 1700–1500 cm�1

This region is characterized by intense transitions in the IR

as well as in the Raman spectrum. The peaks observed close to

1600, 1588, and 1555 cm�1 can easily be assigned to the

fundamentals n13, n15, and n17 (Table 2). The fundamental n13 is

a phenylic mode predominantly localized on ring B. It tends to

be weak in absorption but very strong in Raman. In the case of

the oxygen analogue DBM, the corresponding mode is

probably responsible for the deep Evans transmission window

observed close to 1580 cm�1 in the absorption spectrum, and

for the very strong, sharp Raman peak observed at this position

[41,43]. The modes n15 and n17 involve O–H bending and C C

stretching. They are both strongly IR active, but only n17 is also

strong in Raman. They are predicted to be essentially ‘‘long

axis’’ polarized (a � 08), consistent with the large K-values

observed for these transitions. As previously discussed [25], the

characteristic pattern of transitions in this region of the IR

spectrum strongly indicates the presence of the e-CCC

configuration, but shows no obvious contributions from other

isomers, such as t-CCC.

3.1.3. The region 1500–940 cm�1

This is a relatively complicated region, especially in the

absorption spectrum. Several bands are broad and diffuse,

particularly between 1400 and 1100 cm�1. The broadening is

probably associated with contributions from O–H in-plane

bending motions. Similar band broadening is characteristic for

the spectra of hydrogen bonded hydroxy-compounds [44]. As a

result of the overlapping band structures, identification of

individual transitions is frequently difficult. The observed K-

values range from 0.38 to 0.55 and are of limited help in the

assignment process. The calculated IR absorption spectrum is

not in strikingly good agreement with the observed transitions

in this region. In particular, the relative intensities are not

always well reproduced. The computed Raman spectrum is in

much better agreement with the observed spectrum which

seems less affected by band broadening effects (Fig. 4).

The assignments suggested in Table 2 are in several

instances tentative, particularly for weak transitions. A number

of strongly Raman active modes can be safely identified, such

as n28, n29, n30, n31, n38, and n41. The latter mode, n41, is a

characteristic phenylic mode giving rise to a very sharp

transition near 1000 cm�1. It is observed also in the IR and

Raman spectra of DBM and related compounds [41] (and in the

spectrum of TDBM photoproduct, see below). The B3LYP/cc-

pVTZ calculation on TDBM predicts strong mixing of

equivalent modes from the two phenyl groups, giving rise to

two near-degenerate modes n41 and n40 delocalized over both

rings A and B.

3.1.4. The region below 940 cm�1

Compared with the region between 1500 and 940 cm�1, the

absorption bands in the low energy part of the spectrum seem

better resolved and the agreement with calculated transitions is

more convincing (Figs. 1 and 2). This can probably be

explained by the lower impact of anharmonic effects in the low

energy region. The K-values vary between 0.16 and 0.53,


Fig. 5. (Top) Absorption spectrum of the photoproduct of TDBM trapped in

argon matrix at 20 K [25]. (Bottom) Vibrational absorption spectrum of the t-

TCC configuration of TDBM computed with B3LYP/cc-pVTZ, scaling and

line-shape as in Fig. 1.

providing definite clues to the assignment of the observed

transitions.

The prominent peaks between 940 and 650 cm�1 tend to

have small K-values equal to 0.20 � 0.05 (Fig. 3). Most of them

can easily be assigned to calculated ‘‘short axis’’ polarized

transitions with large a-values, typically involving CH out-of-

plane bending motions (gCH) with occasional admixture of OH

torsion (gOH) (Table 2). An example is provided by the intense

peak near 760 cm�1 with K = 0.16. It can be assigned to the

predicted fundamental n55 with jaj = 828. The nearby absorp-

tion close to 690 cm�1 can be resolved into two components

with K-values equal to 0.26 and 0.16. They can probably be

assigned to the fundamentals n58 and n59 with jaj = 848 and 788.Very small Raman activities are predicted for the modes n55,

n58, and n59, and they are not clearly observed in the Raman

spectrum.

A different example is provided by the transition observed

close to 570 cm�1 which has a large K-value equal to 0.53. It

can be assigned to the fundamental n64 which is predicted to

be essentially ‘‘long axis’’ polarized, with jaj = 98. The

computed Raman activity of this mode is very low. The weak

Raman feature observed at 567 cm�1 can possibly be assigned

to it.

As shown in Fig. 2, there is fairly good agreement between

observed and predicted transitions in the far-IR region. The

relatively intense band around 480 cm�1 has two closely spaced

maxima which can be assigned to the two near-degenerate

fundamentals n66 and n65. Assignment of the ordering of the

two modes is not straightforward. n65 is predicted at higher

wavenumbers than n66, but application of IR intensity and

Raman activity criteria suggests assignment of the reversed

ordering (Table 2).

3.2. Metastable photoproduct (t-TCC)

In this case, only the isotropic IR absorption spectrum in

argon matrix at 20 K is available [25]. Fortunately, the

agreement between calculated and observed spectra is almost

perfect (Fig. 5). It is noteworthy that in this case, the

transitions predicted within the harmonic approximation are

in excellent agreement with the observed spectrum. The

absence of specific anharmonic effects can be explained by

the assignment of the photoproduct to an ‘‘open’’, non-

chelated species (t-TCC), unaffected by intramolecular

hydrogen bonding (in contrast to the case of the stable

ground state e-CCC configuration). The assignment of all

prominent spectral features is relatively straightforward. In

spite of fewer experimental data, we are thus able give a more

detailed account of the vibrational structure of the photo-

product. The assignment of observed transitions to calculated

fundamentals is given in Table 3.

3.2.1. The region above 1700 cm�1

The most interesting absorption in this region is the weak SH

stretching band observed at 2545 cm�1, in excellent agreement

with the calculated fundamental n12 (Table 3). As previously

discussed, the observation and position of this transition is a

strong indication that the trapped photoproduct of TDBM is a

non-chelated enethiol [25].

3.2.2. The region 1700–1400 cm�1

This region displays a characteristic ‘‘signature’’ pattern of

six absorption peaks. This specific pattern is well reproduced by

the transitions computed for the t-TCC configuration (Fig. 5),

but not by those predicted for other configurations, such as t-

CCC or t-TCT [25]. Evidently, these results strongly support

the assignment of the photoproduct to the t-TCC species [25].

The first strong transition at 1645 cm�1 can be assigned to

the fundamental n13 which involves a combination of C O and

C C stretching motions. The following somewhat weaker

transitions at 1602 and 1583 cm�1 are assigned to the phenylic

modes n14 and n16. They correspond more or less to those

observed near 1600 cm�1 in the spectrum of the ground state e-

CCC configuration. The very strong peak at 1546 cm�1 must be

due to n18 which is largely C C and C–C stretching. The

remaining two transitions at 1491 and 1450 cm�1 are assigned

to n20 and n21, again mainly localized on the phenyl groups. The

intervening fundamentals n15, n17, and n19 contribute to the

intensity in this region, but they are predicted to be relatively

weak and are likely to be hidden under the stronger transitions

(Table S2 in Supplementary data).

3.2.3. The region 1400–1100 cm�1

The spectrum shows several strong and well-resolved

transitions in this region. In contrast to the spectrum of the

e-CCC isomer, no significant band broadening is observed. The

experimental transitions are again well reproduced by the

calculated spectrum (Fig. 5). The strong transition at

1257 cm�1 must be assigned to n28 which is essentially in-

plane bending of the central C–H bond (position 2). Other

prominent peaks are observed at 1346, 1309, 1281, 1216, and

1182 cm�1, and they are easily assigned to the C–H bending

modes n23, n25, n27, n29, and n31.


Table 3

The photoproduct of TDBM: assignment of observed IR transitions to fundamental transitions computed for the t-TCC configuration

Observeda B3LYP/cc-pVTZ

nb Ic nb,d Ic App. mode descriptione

3100–2800 {n1 3139 9 CH s

n2 3131 13 CH s

n3 3123 13 CH s

n4 3122 11 CH s

n5 3117 24 CH s

n6 3113 38 CH s

n7 3111 16 CH s

n8 3103 14 CH s

n9 3102 6 CH s

n10 3094 1 CH s

n11 3093 0 CH s

2545 24 n12 2566 31 SH s

1645 152 n13 1648 186 C O s, C C s

1602 64 n14 1604 48 Skel def, CH b (B)

1583 43 n16 1582 44 Skel def, CH b (B)

1546 458 n18 1535 649 C C s, C–C s, C2H b

1491 71 n20 1490 77 Skel def, CH b

1450 26 n21 1449 12 Skel def, CH b (B)

1346 90 n23 1345 80 C2H b

1337 19 n24 1328 3 CH b

1309 35 n25 1319 30 CH b

1299 8 n26 1300 7 CH b

1281 31 n27 1283 16 CH b

1257 132 n28 1240 355 C2H b, CC s

1216 76 n29 1200 92 CH b, CC s

1182 27 n30 1181 1 CH b (B)

1160 3 n31 1177 58 CH b (B)

1094 n33 1087 4 CH b (B)

1080 n34 1081 6 CH b (B)

1049 77 n36 1046 89 CH b (B), SH b

1035 2 n37 1034 5 CH b (A)

1026 31 n38 1025 40 CH b (B)

1018 15 n39 1015 21 SH b

1001 6 n40 999 1 CH oop b (B)

n41 999 3 Skel def (B)

n42 997 5 Skel def (A)

970 2 n43 978 0 CH oop b (A)

n44 974 2 CH oop b (B)

930 20 n46 937 2 CH oop b (B)

n47 930 5 CH oop b, SH b

919 15 n48 918 27 SH b

834 18 n49 849 5 CH oop b (B)

n50 846 2 CH oop b (A)

n51 843 10 CH oop b (B)

818 4 n52 809 4 Skel def (CS s)

784 16 n53 790 15 CH oop b (B)

759 83 n54 765 70 CH oop b (A)

699 100 n55 703 49 CH oop b (A)

n56 700 51 CH oop b (B)

694 28 n57 693 27 CH oop b (B)

686 7 n58 686 10 Skel def

672 26 n59 671 31 Skel def

609 17 n60 623 3 Skel def (A)

n61 620 1 Skel def (B)

592 3 n62 598 6 Skel def

566 33 n63 565 40 Skel def, SH b

489 19 n64 495 21 Skel def, C-SH tor (gSH)

a Argon matrix at 20 K [25].b n, wavenumber (cm�1).c I, integrated intensity relative to I(n55) + I(n56) = 100.d Scaling factor, 0.977.e See Table 2, footnote j.


3.2.4. The region 1100–900 cm�1

In comparison with the other transitions in the spectrum, the

band at 1094 cm�1 is curiously broad. Overlap with the nearby

transition at 1084 cm�1 may influence the shape of the band,

but otherwise we have no obvious explanation for the

broadening. The two near-degenerate transitions are best

assigned to the modes n33 and n34, but the predicted intensity of

these transitions seems too low to account for the observed

absorption. The following four peaks at 1049, 1026, 1018, and

1001 cm�1 are well accounted for by the calculated modes n36,

n38, n39, and n41, although several additional, less active modes

probably contribute to the observed intensity. The extremely

sharp transition at 1001 cm�1 corresponds to similar, very sharp

transitions near 1000 cm�1 in the spectra of DBM [41] and the

stable e-CCC configuration of TDBM (see above). The two

peaks at 930 and 919 cm�1 can be assigned to n47 and n48 which

involve S–H in-plane bending, but the relative intensity of the

two transitions is not well predicted.

3.2.5. The region below 900 cm�1

Several transitions, such as those at 834, 784, 759, 699, and

694 cm�1, can be assigned to C–H out-of-plane bending

vibrations, gCH. The broadness of the band at 834 cm�1 can be

explained by overlapping contributions from the near-

degenerate gCH modes n49, n50, and n51. Also the strong,

composite band peaking at 699 cm�1 must be due to several

gCH modes, including n55, n56, n57, and n58.

4. Concluding remarks

The observed absorption spectra for the stable ground state

species of TDBM indicates the significance of anharmonic

effects associated with strong intramolecular hydrogen bond-

ing. The expected O–H stretching band was not clearly

observed, and parts of the spectrum were complicated by

overlapping, broad band structures. The analysis was supported

by the observed polarization data and by the Raman spectrum,

which was much less affected by band broadening effects. As a

result, a large number of observed transitions could be assigned

to fundamentals predicted with B3LYP/cc-pVTZ for the e-CCC

configuration. No obvious indication of contributions to the

observed spectra from other configurations than e-CCC could

be found.

Assignment of the observed absorption bands for the

photoproduct of TDBM to those predicted for the t-TCC

configuration was relatively straightforward. In fact, the

spectrum predicted with B3LYP/cc-pVTZ provided a near-

perfect match of the observed spectrum. For example, the weak

S–H stretching band observed near 2550 cm�1, and the pattern

of transitions in the ‘‘signature region’’ 1700–1400 cm�1 were

accurately reproduced by the calculation.

The present detailed analyses of the vibrational spectra of

TDBM and its photoproduct confirm the previous assignment

of the stable ground state species to the chelated-enol form (e-

CCC) and the metastable photoproduct trapped in argon matrix

to the non-chelated enethiol configuration (t-TCC) [25].

Equivalent conclusions have been reached for thioacetylace-

tone [26], p-methyl(thiobenzoyl)acetone [27], and p-methyl-

benzoylthioacetone [45], thus indicating a common mechanism

for the photoreactivity of b-thioxoketones.

Acknowledgements

We are grateful to the European Union, the Danish Natural

Science Research Council, and Roskilde University for

financial support and to Ole Faurskov Nielsen and his staff

at the University of Copenhagen for measuring the Raman

spectrum of TDBM.

Appendix A. Supplementary data

Supplementary data associated with this article can be

found, in the online version, at 10.1016/j.vibspec.2006.06.015.

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Monothiodibenzoylmethane: Structural and vibrational assignments

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