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Matrix and ab initio infrared spectra of thiourea and thiourea-d4
Vibrational Spectroscopy 18 Ž1998. 91–102
Matrix and ab initio infrared spectra of thiourea and thiourea-d 4 L. Bencivenni
a,b
, S. Nunziante Cesaro
c,)
, A. Pieretti
a,b
a
c
Dip. Chimica, UniÕersita’ ‘La Sapienza’, 00185, Rome, Italy b Istituto Nazionale di Fisica della Materia (INFM), Italy C.S. per la Termodinamica Chimica alle Alte temperature, c r o Dip. Chimica, UniÕersita’ ‘La Sapienza’, P.le A. Moro, 5, 00185, Rome, Italy Received 15 April 1998; revised 12 September 1998; accepted 16 September 1998
Abstract The FTIR spectrum of thiourea and thiourea-d 4 isolated in argon and nitrogen matrices are presented for the first time and discussed in terms of normal modes predicted by ab initio calculations. Vibrational frequencies, IR intensities and isotopic shifts obtained from the density functional B3-LYPr6-31q GŽ2d,p. calculations show the most satisfactory agreement with experiments. In contrast with previous results, the present theoretical study indicates non-planarity of the molecule. The lowest energy structure has C 2 symmetry and the less stable isomers has C s symmetry. Experimental data clearly indicate the presence of a single isomer in the vapor. The decomposition of thiourea for vaporization temperature over 410 K is also discussed. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Vaporization temperature; IR intensities; Isotopic shifts; Thiourea-d 4 ; Vibrational frequencies
1. Introduction The vibrational spectrum of thiourea has continued to be studied since the first paper by Edsall in 1937 w1x. The first tentative assignment of thiourea molecule in alkali halide pellets was reported by Stewart w2x. The assignment for in plane vibrations has been supported by the normal coordinate treatments of thiourea and thiourea-d 4 by Yamaguchi et al. w3x and Hadzi et al. w4x who investigated also the infrared spectra of thiourea in nujol and CH 3 CN
) Corresponding author. Tel.: q39-6-49913962; Fax: q39-649913951; E-mail: [email protected]
solution and included in the work hydrogen and nitrogen isotopically substituted analogues. Further support came from the work of Peyronel et al. w5x on thiourea dry powder spread over KBr pellets. In plane and out of plane fundamentals of thiourea have been calculated using a general force field by Aitken et al. w6x and Arenas et al. w7x, respectively. Infrared polarized spectra of single crystals of thiourea have been analyzed long ago in the N–H stretching region only w8x. A complete analysis of single crystal of thiourea has been given by Schrader et al. w9x and Bleckmann et al. w10x both in infrared and Raman. Quite recently, spectra, geometry and force field of thiourea and its deuterated and 15 N isotopomers have been the object of an ab initio extensive study w11x. In all the aforementioned works, bands are assigned assuming a C 2v symmetry for the thiourea molecule.
0924-2031r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 3 9 - 3
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L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
It is worth noticing that thiourea adsorbed on roughened silver has recently attracted renewed interest because of its surface enhanced Raman scattering ŽSERS. behavior depending on complexation, protonation and co-adsorption w12x. In this context, the interaction between the molecule and silver atoms present some interest. Therefore, we have undertaken a systematic investigation on the formation of the silver–thiourea complex using the matrix isolation technique. Since spectroscopic data on the free molecule are lacking in the literature, the first step of the work necessarily involved the complete characterization of the isolated thiourea. In this work spectra of argon and nitrogen isolated thiourea and thiourea-d 4 are therefore presented and discussed in terms of potential energy distribution analysis predicted by ab initio calculations.
2. Experimental The experimental apparatus basically consists of a cryotip ŽAir Products CSA202. connected under rotary vacuum to a Bruker Interferometer IFS 113 v through a suitable IR transparent window ŽCsI or polyethylene.. The gold plated copper cold finger is free to rotate in a homemade shroud under high vacuum Ž10y6 Torr.. The isolating gases Žargon or nitrogen of high purity supplied by Caracciolo. reached the cold finger through a stainless steel inlet equipped with a standardized needle valve. A flow rate of 1 mmolrh was needed for a good isolation degree. Thiourea ŽICN Biochemicals, 99%. and thiourea-d 4 Ž98% enriched, Aldrich. were vaporized in the 350 to 390 K temperature range Ž10y7 –10y6 atm.. Over 410 K, the molecule partly decomposes in ammonia and isothiocyanic acid. Deposition times lasted from 15 to 90 min. Spectra were taken in reflection, accumulating at least 200 scans at a resolution of 1 cmy1 or better. Annealing cycles were normally performed rising the temperature of the matrix up to 30 K and recooling.
3. Vibrational assignment The FTIR spectra of argon and nitrogen thiourea and deuterated analogues, obtained under identical experimental conditions, looked always very similar showing only frequency shifts due to the different environment. Therefore, in Figs. 1 and 2 are reported only spectra of argon isolated molecules, including features observed under 200 cmy1 where nitrogen looses its transparency. Experimental frequency values of fundamental modes of thiourea and thiourea-d 4 are summarized in the Tables 1 and 2. Only the internal coordinates having a strong contribution in each mode appear in the assignment while the complete picture is reported in Section 4.2 ŽTable 3.. The assignment is proposed in terms of a non-planar C 2 symmetry according to the calculations reported in Section 3.1. So far, this finding is in contrast with the assumption of planar geometry universally adopted in the literature but agrees with recent ab initio results on urea molecule w13x. For sake of simplicity the observed peaks and assignment are discussed separately for each spectral region. 3.1. N–H stretching modes In the upper frequency region ŽFig. 1a. the N–H stretching modes are expected. In argon isolated thiourea, in fact, four peaks have been detected: a strong absorption is shown at 3544.9 cmy1 , a medium intensity doublet at 3431.6r3423.0 cmy1 and two weak bands at 3682.0 cmy1 and 3494.9 cmy1 . Corresponding features in nitrogen appear at 3540.8, 3429.2r3420.3, 3663.5 and 3493.1 cmy1 , with approximately the same intensity ratio. In both media, the intensity of the features observed is unaffected upon annealing cycles. The excellent agreement between calculated and observed intensities confidently suggests to assign the bands at 3682.0 and 3494.9 cmy1 to the symmetric stretching modes of A type and peaks at 3544.9 and 3431.6r3423.0 cmy1 to the B type asymmetric stretchings Žsee Table 1.. Since combination bands and overtones are not expected in this spectral region, the splitting of the asymmetric
Fig. 1. The FTIR spectrum of argon isolated thiourea Ža., N–H stretching region Žb., NH 2 bending and C–N stretching region Žc., NH 2 rocking and C5S stretching region Žd., low frequency mode.
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
93
94
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
Fig. 2. The FTIR spectrum of argon isolated thiourea-d 4 Ža., N–D stretching region Žb., ND 2 bending and C–N stretching region Žc., ND 2 rocking and C5S stretching region Žd., low frequency modes.
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102 Table 1 IR experimental frequencies Žcmy1 . of argon and nitrogen isolated thiourea Thiourea in Ar
Thiourea in N2
Assignment a
3682.0 3544.9 3494.9 3431.6r3423.0 1613.8 1592.9 1412.7 1391.4 1053.8 1035.9 758.9 307.5 291.5 281.5
3663.5 3540.8 3493.1 3429.2r3420.3 1617.7 1594.9 1419.3 1393.4 1059.4 1034.4 753.8
ns N–H ŽA. nas N–H ŽB. ns N–H ŽA. nas N–H ŽB. d NH 2 ŽA. d NH 2 ŽB. nas C–N ŽB. ns C–N ŽA. r NH 2 ŽB. r NH 2 ŽA. n C5S ŽA. d C5S ŽB. v NH 2 ŽB. t NH 2 ŽA.
a
Fundamental modes are classified according to C 2 group of symmetry Žsee text.. n:stretching, d:bending, r:rocking, v:wagging, t:torsion. Symmetry characters of the modes are given in parentheses.
stretching Ž3431.6r3423.0 cmy1 in argon and 3429.2r3420.3 cmy1 in nitrogen. in both the environment still remains unexplained. In the same spectral region, water traces are readily identified at 3731.3 and 3642.0 cmy1 in argon and 3726.4 and 3634.5 cmy1 in nitrogen, respectively. It is worth observing that frequency values of N–H stretching modes of free thiourea are significantly higher than those of the corresponding modes of solid thiourea Ž3390–3090 cmy1 . w1,2,4,8–10x and those of thiourea in CH 3 CN solution Ž3473–3358 cmy1 . w4x. The continuous blue shift in the three states Žsolid, solution and free molecule. is attributed to the weakening of the hydrogen bond effect, passing from solid to solution states, until the complete loss of coupling in the isolated molecule. Literature data concerning isotopic derivatives of thiourea are quite poor. Hadzi et al. w4x reported the N–D stretching modes of thiourea-d 4 in solid and solution states in the range 2540–2360 cmy1 and 2605–2444 cmy1 , respectively. In the spectral region where N–D stretching modes are expected, argon isolated thiourea-d 4 showed four bands at 2621.5, 2500.5, 2491.2 and 2464.5 cmy1 . The corresponding features in nitrogen appeared at 2629.8, 2503.2, 2494.2 and 2472.9 cmy1 . N–D stretching
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modes appear sensibly weaker than corresponding N–H modes, and show a slight different intensity ratio. For this reason, the correspondence proposed in the table, grounded only on the calculated values, must be considered tentative. 3.2. NH2 bending modes (d NH2 ) Symmetric and asymmetric bending modes Ž d NH 2 . of thiourea coincide in a single broad band, lying in the tight interval 1617–1590 cmy1 , both in solid w2,4,14x and solution states w4x. Only dry powdered thiourea samples spread over KBr pellets w5x showed two distinct peaks at 1612 and 1590 cmy1 . The complete deuteration of the molecule, increased the frequency difference between the two modes allowing the observation of both of them at 1189 and 1141 cmy1 in solid w4x and 1199 and 1140 cmy1 in CH 3 CN solution w4x. Normal coordinate analysis, available in the literature, appears quite contradictory. Yamaguchi, in fact, predicted A and B type d NH 2 at 1623 and 1619 cmy1 , respectively, and the corresponding d ND 2 at 1204 and 1188 cmy1 , in close agreement with experimental data w3x. In contrast, Aitken et al. calculated symmetric and asym-
Table 2 IR experimental frequencies Žcmy1 . of argon and nitrogen isolated thiourea-d 4 Thiourea-d 4 in Ar
Thiourea-d 4 in N2
Assignment a
2621.5 2500.5 2491.2 2464.5 1431.0 1353.9 1276.7 1127.4 879.4 811.9 710.1 255.9 229.4 211.2
2629.8 2503.2 2494.2 2472.9 1449.3 1355.8 1284.2 1131.7
n s N–D ŽA. n as N–D ŽB. ns N–D ŽA. nas N–D ŽB. nas C–N ŽB. ns C–N ŽA. d ND 2 ŽA. d ND 2 ŽB. r ND 2 ŽA. r ND 2 ŽB. n C5S ŽA. d C5S ŽB. v ND 2 ŽB. t ND 2 ŽA.
a
Fundamental modes are classified according to C 2 group of symmetry Žsee text.. n:stretching, d:bending, r:rocking, v:wagging, g:out of plane, t: torsion. Symmetry characters of the modes are given in parentheses.
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L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
metric d NH 2 at 1634 and 1625 cmy1 but sensibly higher frequency values for corresponding d ND 2 Table 3 B3-LYPr6-31qGŽ2d, p. harmonic frequencies of vibration Žcmy1 . and potential energy distribution of the most stable C 2 symmetry isomers of thiourea and thiourea-d 4 Vibrational frequencies Žcmy1 . a Thiourea 3702 Ž26. 3701 Ž68. 3571 Ž13. 3563 Ž48. 1658 Ž74. 1632 Ž211. 1427 Ž315. 1411 Ž315. 1072 Ž19. 1068 Ž73. 766 Ž12. 636 Ž4. 580 Ž91. 497 Ž40. 458 8Ž4. 401 Ž4. 386 Ž316. 337 Ž128. Thiourea-d 4 2749 Ž21. 2737 Ž32. 2581 Ž16. 2574 Ž52. 1445 Ž277. 1376 Ž428. 1202 Ž29. 1157 Ž12. 895 Ž17. 834 Ž5. 713 Ž10. 630 Ž6. 416 Ž47. 401 Ž13. 379 Ž19. 367 Ž10. 292 Ž162. 244 Ž64. a b
Potential energy distributionb
ŽA. ns N–H Ž96. ŽB. nas N–H Ž96. ŽA. ns N–H Ž95. ŽB. nas N–H Ž96. ŽA. d NH 2 Ž92. ŽB. d NH 2 Ž87.q nas C–NŽ12. ŽB. nas C–N Ž70.qr NH 2 Ž12.q n C5S Ž10. ŽA. r NH 2 Ž37.q ns C–N Ž24.q n C5S Ž20.q v NH 2 Ž16. ŽB. r NH 2 Ž84.q nas C–N Ž15. ŽA. ns C–N Ž53.qr NH 2 Ž35.q n C5S Ž11. ŽA. n C5S Ž66.q ns C–N Ž17. ŽB. g C5S Ž91. ŽB. t NH 2 Ž81.q v NH 2 Ž16. ŽA. t NH 2 Ž42.q v NH 2 Ž38.qd NCN Ž10. ŽA. d NCN Ž76.qr NH 2 Ž13. ŽB. d C5S Ž91. ŽB. v NH 2 Ž62.q t NH 2 Ž24.q ns C–N Ž13. ŽA. t NH 2 Ž42.q v NH 2 Ž38.qd NCN Ž10.
ŽA. ns N–D Ž97. ŽB. nas N–D Ž98. ŽA. ns N–D Ž97. ŽB. nas N–D Ž97. ŽB. nas C–N Ž77.qd ND 2 Ž11. ŽA. ns C–N Ž44.q n C5S Ž19.qd ND 2 Ž15.q d NCN Ž12.qrND 2 Ž10. ŽA. d ND 2 Ž67.q n C5S Ž12.qr ND 2 Ž11.q d NCN Ž10. ŽB. d ND 2 Ž88. ŽA. ns C–N Ž44.qr ND 2 Ž41.qd ND 2 Ž10. ŽB. r ND 2 Ž85. ŽA. n C5S Ž68.qr ND 2 Ž15.q ns C–N Ž13. ŽB. g C5S Ž87. ŽB. t ND 2 Ž47.q v ND 2 Ž16. ŽB. d NCN Ž58.q v ND 2 Ž27.qr ND 2 Ž15. ŽA. v ND 2 Ž46.qd NCN2 Ž20.q t ND 2 Ž17.q r ND 2 Ž10. ŽB. d C5S Ž79.qr ND 2 Ž11. ŽB. v ND 2 Ž61.q t ND 2 Ž27.q nas C–N Ž11. ŽA. t ND 2 Ž49.q v ND 2 Ž35.
IR intensities ŽKm moly1 . are reported in parentheses. P.E.D. contribution lower than 10% are not reported.
Ž1388 and 1507 cmy1 . w6x. A more recent ab initio investigation placed the asymmetric d NH 2 at 1622 cmy1 Ž1159 cmy1 for d ND 2 . and the symmetric one at 1657 cmy1 Ž1200 cmy1 for d ND 2 . with approximate intensity ratio 3:1 w11x. Our theoretical analysis predicts the B and A type d NH 2 at 1632 and 1658 cmy1 with intensity ratio 3:1. The corresponding vibrations of thioures-d 4 are calculated at 1157 and 1202 cmy1 , respectively. According to our theoretical treatment both bending modes of thiourea-d 4 should be much less intense than corresponding modes of thiourea. In addition, the symmetric bend of thiourea-d 4 is expected twice more intense than the asymmetric one, with a trend opposite to that shown by the corresponding vibrations of thiourea. Two intense features, at 1613.8 and 1592.9 cmy1 in argon and 1617.7 and 1594.9 cmy1 in nitrogen, have been assigned to the symmetric and asymmetric d NH 2 ŽFig. 1b., since they match the required spectral position and intensity ratio inferred from literature data and our theoretical results. In both the environment, the two fundamental modes embed the bending mode of water Ž1599.6 and 1597.9 cmy1 in argon and nitrogen, respectively. which could not be eliminated from the vaporization of the sample. Corresponding modes of argon isolated thiourea-d 4 have been individuated at 1276.6 and 1127.4 cmy1 ŽFig. 2b. for two main reasons: their closeness to experimental frequency values of d ND 2 in solid and solution states w4x and to values calculated by ab initio treatment Žw11x and this work.. In addition, the intensity ratio of the mentioned bands mirrors quite well the ratio predicted in this work Žsee Table 3.. In nitrogen environment the d ND 2 are detected at 1284.2 and 1131.7 cmy1 and appear weaker than in argon, as observed for the N–H stretching modes, but still keep the predicted intensity ratio. 3.3. The 1400–1000 cm y 1 region In the 1400–1000 cmy1 spectral region four fundamental modes are expected and are due to different contributions of C–N stretching, NH 2 rocking Žr NH 2 . and C5S stretching in minor extent. Literature data show a substantial agreement in assigning two intense peaks around 1400 cmy1 to the C–N stretching mode and two bands of medium
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
intensity around 1000 cmy1 to the r NH 2 w2,4,6x. However, in the study of dry powdered thiourea the rocking modes were placed at higher frequency values Ž1210 and 1210 cmy1 . w5x. The attribution of the C–N stretching modes of thiourea-d 4 was not settled in early literature and the frequency value of these bands ranged from 1400 to 1100 cmy1 w4,6x. The rocking modes of the ND 2 group were however individuated in the 900–800 cmy1 interval w4x. Frequency values and intensity of bands having a strong C–N contribution resulting from the theoretical treatment presented in this work ŽTable 3. match fairly well previous ab initio results w11x. On the other hand, the assignment of the NH 2 rocking modes shows some discrepancy. In fact, the asymmetric r NH2 is located at the high frequency side of the symmetric one Ž1072 and 1068 cmy1 , respectively. in contrast with the previous evaluation Žasymmetric r NH 2 at 1051 cmy1 and symmetric r NH 2 at 1055 cmy1 . w11x. In argon isolated spectra of thiourea, two bands are shown at 1412.7 and 1391.4 cmy1 Ž1419.1 and 1393.4 cmy1 in nitrogen. ŽFig. 1b.. The spectral position and the intensity ratio of these peaks strongly suggest their assignment to modes heavily involving the C–N stretching of B and A type, respectively. Upon complete deuteration the corresponding features are individuated at 1431.0 and 1353.9 cmy1 in argon and 1449.3 and 1355.8 in cmy1 nitrogen. The increase of the spread between them and the decrease of their intensity ratio are in excellent agreement with theoretical predictions Žthis work and Ref. w11x., thus giving further support to the attribution. In matrix isolated thiourea spectra, bands attributable to modes with strong contribution of the NH 2 rocking are observed at 1053.8 and 1035.9 cmy1 in argon ŽFig. 1c. and 1059.4 and 1034.4 cmy1 in nitrogen ŽTable 1.. In argon, both bands have approximately the same intensity while in nitrogen the high frequency peak appears of double intensity with respect to the remaining one. The marked sensitivity to the environment is not surprising on consideration of the specific interaction of the N–H bond toward the N2 molecule. The same explanation could apply to the absence of corresponding modes in nitrogen isolated thiourea-d 4 . In argon, however, both modes are clearly detected at 879.4 and 811.9
97
cmy1 ŽFig. 2c. having approximately the intensity ratio predicted in the theoretical treatment presented here Žsee Section 4.2 and Table 3.. 3.4. The 1000–200 cm y 1 region In this region the C5S stretching and the C5S out of plane bending modes are expected in the 800–600 cmy1 interval, along with six further fundamentals under 600 cmy1 containing the participation of the C5S and N–C–N bending and the torsion Ž t NH 2 . and wagging modes of the NH 2 group Ž v NH 2 .. A band lying around 730 cmy1 observed in spectra of thiourea in solid, solution or dry powder samples has been assigned to the C5S stretching w4,5x with the exception of Stewart who attributed an intense peak appearing at 730 cmy1 in solid thiourea spectra to the C–N stretching w2x. In totally deuterated samples the band shifted about 50 toward the red w4x. Normal coordinate analysis w6x and former ab initio w11x calculation support the mentioned frequency values. The theoretical treatment presented here provides 766 and 713 cmy1 for the C5S stretching of thiourea and thiourea-d 4 , respectively, in good agreement with the experimental values of argon isolated thiourea and thiourea-d 4 at 758.9 and 710.1 cmy1 , respectively ŽFig. 1cFig. 2c.. In nitrogen, the corresponding feature of thiourea was observed at 753.8 cmy1 but, unfortunately, the C5S stretching of thiourea-d 4 was not detected even in concentrated spectra. The individuation of modes lying under 600 cmy1 in literature data is far from exhaustive: an intense peak detected at 486 cmy1 by several authors in solid thiourea samples w2,4,5x has been mainly assigned to the N–C–N bend and a medium intensity band at 411 cmy1 has been attributed to NH 2 torsion w2x or N–C5S bend w4x. The latter hypothesis finds some support in the normal coordinate analysis reported by Aitken et al. w6x. The frequency values resulting from our theoretical treatment are collected in Table 3, together with the proposed assignment. It is however worth noticing that calculated intensities are quite different from those proposed in previous ab initio analysis w11x. In the 600–30 cmy1 region, argon isolated thiourea shows three bands at 307.5, 291.5 and 281.5 cmy1 ŽFig. 1d.. Upon complete deuteration the three
98
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
bands shift at 255.9, 229.4 and 211.2 cmy1 . Corresponding features in nitrogen have not been observed due the loss of intensity of this matrix in the far infrared region. Since the agreement between experimental and theoretical values is quite poor in this spectral range, the assignment proposed in Tables 1 and 2 to fundamental modes of thiourea and thiourea-d 4 , respectively, must be considered only tentative.
4. Computational study The Gaussian 94 package programs w15x was used for running the ab initio and density functional calculations reported in this work. The geometry optimizations of the structures of thiourea were carried out at the second order of the Moller–Plesset theory including all electrons in the correlation calculation and using the B-LYP and B3-LYP density functionals w16–18x. The DZŽd,p. and the 6-31 q GŽ2d,p. basis sets w19x stored in the Gaussian 94 program w15x, were chosen for the study. The scanning of the energy surface of thiourea was performed at the second order of the Moller–Plesset level considering all electrons active with the double-zeta ŽDZ. basis set augmented with multiple polarization functions.
These calculations are indicated in the work as MP2rDZŽd,p., MP2rDZŽ2df,2p. and MP2rDZ Ž3df,2p.. A further stability study was carried out at the fourth order of the Moller–Plesset theory within the frozen-core approximation MP4rDZŽd,p.. The single-point energy computations were performed on the MP2rDZŽd,p. optimized geometry of thiourea. 4.1. Structures In this section of the study we report the main results accomplished from the computations. The Moller–Plesset and the B3-LYP density functional calculations indicate that there are two stable non-planar isomers of the molecule. The lowest energy structure has C 2 symmetry ŽFig. 3a. and the less stable isomer has C s symmetry ŽFig. 3b.. Nonplanarity of these isomers is due to the NH 2 groups of the molecule. On the contrary, the N–CS–N moiety of both the C 2 and C s symmetry structures is planar. For this reason, thiourea and urea w13x are isostructural molecules. The present theoretical study indicates without any doubt non-planarity of these isomers and therefore there is not agreement with the previous theoretical conclusions w11x which attributed to thiourea molecule a planar structure of C 2v symmetry. Such discrepancy is likely due to the
Fig. 3. Non-planar lowest energy structures of thiourea: Ža. C 2 symmetry isomer, Žb. C s symmetry isomer and Žc. C 1 symmetry transition state. Higher energy structures: Žd. planar C 2v symmetry, Že. non-planar C s symmetry and Žf. and Žg. non-planar C 2v symmetry structures.
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
use of the 3-21G) basis sets w11x. The results of the single-point energy calculations are summarized in Table 4 and indicate that the C 2 symmetry isomer is more stable than the C s symmetry one within a few kJ moly1 . In addition to this, the isomerization barrier is quite low so that the conversion of the C s isomer into the C 2 symmetry one is straightforward. From the theoretical data determined for the C 2 and C s symmetry isomers one might in fact conclude that the most stable C 2 isomer is nearly exclusively present in the vapor at the vaporization temperature of the FTIR measurements. This is indeed quite important for the interpretation of the FTIR spectrum of matrix isolated thiourea. The most stable isomers are separated by the transition state shown in Fig. 3c having one planar NH 2 group. The remaining structures of thiourea which were taken into account are lying in the high energy region of the energy surface and are second order saddle pints. Among them, the C 2v symmetry one shown in Fig. 3d has planar structure and is 15 kJ moly1 unstable with respect to the most stable isomer. This is an interesting result because it shows without any doubt that the planar C 2v symmetry structure, indicated elsewhere as the stable equilibrium structure of thiourea w11x, is not the stable minimum of the molecule. The other non-planar models are the C s ŽFig. 3e. and the C 2v ŽFig. 3f and g. symmetry and are lying above 100 kJ moly1 with respect to the lowest energy minimum isomer. Evi-
99
dently these structures, which are all second order saddle points, have no practical interest, being unstable forms. The summary of the calculations is given in Table 4 Žstability. and Table 5 Žgeometries and harmonic vibrational frequencies.. The results of our theoretical study on thiourea molecule suggests a close similarity with quite recent conclusions obtained for urea molecule w20,21x. 4.2. Theoretical Õibrational assignment The theoretical vibrational spectrum of the most stable isomer of thiourea is important for understanding the matrix isolation study of the free molecule. The MP2 and density functional calculations of the C 2 symmetry isomer provide nearly identical IR patterns. However, the best theoretical result was obtained from the density functional B3-LYPr6-31 q GŽ2d,p. calculation which shows the most satisfactory agreement with the experiments. We have also determined the potential energy distribution of this isomer from the MP2 and the density functional frequencies of vibration and both these computational methods provide the same conclusions, that is the same theoretical assignment of the calculated harmonic vibrational frequencies. The potential energy distribution was determined by using a computer program written by one of the authors. This program employs the cartesian force constants supplied from the Gaussian 94 program
Table 4 Relative energies Ž D E, kJ moly1 . calculated with respect to the C 2 symmetry isomer of thiourea MP2rDZ Žd,p.
MP2rDZ Ž2df,2p.
MP2rDZ Ž3df,2p.
MP4rDZ Žd, p.
DZPE a
Stable isomers C2 Cs
0 4
0 3
0 5
0 5
0 y2
Transition state C1
5
3
5
6
y3
8
9
High energy second order saddle point C 2v 6 Cs 126 C 2v 136 C 2v 142 a
Zero-point energy difference calculated at the MP2rDZŽd,p. level.
100
Table 5 Optimized equilibrium geometries Ža., harmonic frequencies Žcmy1 . Žb.and IR band intensities Žc. of the C 2 and C s symmetry isomers of thiourea Geometriesa
b
Cs
C2
Cs
C2
C 2 symmetry
MP2r DZŽd,p.
MP2r DZŽd,p.
B-LYPr DXŽd,p.
B-LYPr DXŽd,p.
B-LYPr 6-31qG Ž2d,p.
MP2r DZŽd,p.
B-LYPr DZŽd,p.
B3-LYPr 6-31qG Ž2d,p.
Symmetry
MP2r DZŽd,p.
B-LYPr DZŽd,p.
Symmetry
1.657 1.377 1.008 1.009 123.0 113.8 119.9 115.0 116.3 23.5 169.9 y161.8 y15.4
1.682 1.394 1.020 1.023 123.2 113.6 117.8 117.8 114.9 29.8 168.5 y150.2 y11.5
1.686 1.388 1.018 1.021 5 122.6 114.7 114.95 114.95 116.7 22.1 170.8 y162.4 y13.7
1.6695 1.365 1.008 1.011 122.6 5 114.7 115.5 119.8 5 116.3 23.1 170.3 y156.9 y9.7
3781 3781 3645 3641 1689 1669 1460 1430 1102 1100 799 649 612 587 461 438 401 391
3595 3595 3460 3543 1597 1573 1364 1360 1038 1031 737 610 559 562 Ž91. 435 414 383 366
3702 Ž26. 3701 Ž68. 3571 Ž13. 3563 Ž48. 1658 Ž74. 1632 Ž211. 1411 Ž315. 1427 Ž103. 1072 Ž19. 1068 Ž73. 766 Ž12. 636 Ž4. 497 Ž40. 580 Ž91. 458 Ž4. 401 Ž4. 386 Ž316. 337 Ž128.
A B A B A B A B B A A B A B A B B A
3809 3807 3667 3659 1697 1664 1460 1448 1090 1068 803 622 578 464 438 412 394 171
3622 Ž12. 3620 Ž59. 3479 Ž7. 3467 Ž23. 1600 Ž73. 1564 Ž189. 1361 Ž238. 1372 Ž99. 1025 Ž48. 1005 Ž18. 740 Ž8. 593 Ž4. 553 Ž49. 437 Ž4. 423 Ž59. 362 Ž396. 378 Ž1. 128 Ž49.
A Y A X A Y A X A Y A Y A Y A X A Y A X A X A X A X A Y A X A Y A Y A
Bond lenghts ŽA., interatomic angles Ž8., dihedral angles Ž8.. cmy1 . c IR intensities Žkcal moly1 .. X H is the H atom of the longest N–H bond distance. b
C s symmetry
X
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
C2
C5S 1.652 C–N 1.384 N–H 1.0095 X N–H 1.011 5 N–C–S 123.8 N–C–N 112.4 H–N–C 116.4 X H –N–C 115.5 X H–N–H 113.5 X N–C–N–H 30.9 N–C–N–H 167.3 X S–C–N–H y149.1 S–C–N–H y12.7
a
Vibrational frequencies and IR band intensities c
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
frequency calculations, carried out for the optimized geometry and the non-redundant set of internal coordinates. The B3-LYPr6-31q GŽ2d,p. vibrational frequencies, along with the calculated IR band intensity, and the potential energy distribution of C 2 symmetry thiourea and thiourea-d 4 were quite useful for the assignment of the matrix isolated spectra, as widely discussed in Section 4.1. The last observation to be discussed concerns the expected IR spectrum of the less stable isomer of C s symmetry. As it may be seen in Table 5, the vibrational spectrum of this species might be sufficiently different from that of the most stable and predominant isomer having C 2 symmetry and therefore, one might be able to check experimentally the presence of the less stable form. The fact that the matrix isolated spectra show no evidence of the C s symmetry isomer seems to suggest that the vapor phase consists of the most stable C 2 symmetry molecule exclusively.
5. Decomposition products Long-lasting depositions at different vaporization temperature from 350 to 420 K were performed in order to obtain concentrated samples for a safe characterization of weakest bands of thiourea. According to thermodynamic measurements, thiourea does not show appreciable decomposition before the melting point and totally decomposes during the transition phase w22x. On deposition over 410 K, low intensity bands were detected. The attribution of these bands to the thiourea molecule was ruled out due to the different grow rate with the vaporization temperature. Infrared data cannot give quantitative indication on the decomposition extent. However, the percentage of decomposed molecules should not exceed the 10% of the total amount in the detection limit of the thermodynamic measurement. Decomposition products were obviously observed in the matrix isolated infrared spectra owing to the high sensitivity of this technique. Thiourea should produce isothiocyanic acid ŽHNCS. and ammonia, in analogy with urea which
101
gives HNCO and NH 3 w23x. The presence of ammonia was readily proved by the appearance of its most intense features at 3446. and 974.6 cmy1 , in argon matrices, assigned to n4 and n 2 , respectively, in excellent agreement with literature data Žw24x.. Only the strongest peak at 969.7 cmy1 was observed in nitrogen. A very weak band at 3400.7 cmy1 in argon suggested the presence of traces of ammonia dimer w24x. The formation of HNCS was suggested by the appearance of a doublet at 1996.8r1981.9 and 1996.6r1991.6 cmy1 in argon and nitrogen, respectively, which has been assigned to the C–N stretching mode of this molecule on ground of literature data w25x. On vaporization of thiourea-d 4 over 410 K, some DNCS was formed having the C–N stretch at 1938.7 cmy1 , observed only in argon moiety w25x. Remaining fundamental bands of HNCS and DNCS were never detected in our spectra in agreement with the weaker intensity expected w25x. MP2 and density functional computations gave further support to the assignment of the bands observed to the isothiocyanic acid. For the sake of brevity, the computational details will not be discussed here. Shortly, the treatment predicts the N–H, C5N and C5S stretching vibrations at 3565, 1953 and 712 cmy1 , respectively, and indicates a high intensity in the infrared for the C5N stretching mode as actually observed experimentally.
6. Conclusion Comparison of the spectra of thiourea and thiourea-d 4 in solid, solution and pseudo gaseous states clearly shows marked differences in the region of the N–H stretchings, due to the different efficiency of the hydrogen bonding. Spectra of matrix isolated thiourea and the deuterated analogue are in excellent agreement with calculations, suggesting a non-planar C 2 symmetry, in contrast with previous conclusions. Some discrepancy is however present between experimental and theoretical data in the far infrared region. According to the theoretical treatment, the barrier between the lowest energy state of C 2 symmetry and the less stable isomer of C s symmetry should be
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quite low. However, experiments suggest the presence of a single isomer in the vapor.
References w1x J.T. Edsall, J. Phys. Chem. 41 Ž1937. 133. w2x J.E. Stewart, J. Chem. Phys. 26 Ž1967. 248. w3x A. Yamaguchi, R.B. Penland, S. Mizushima, T.J. Lane, C. Curran, J.V. Quagliano, J. Am. Chem. Soc. 80 Ž1958. 527. w4x D. Hadzi, J. Kidric, Z.V. Znezevic, B. Barlic, Spectrochim. Acta 32A Ž1976. 693. w5x G. Peyronel, A. Pignedoli, W. Malavasi, Spectrochim. Acta 40A Ž1984. 63. w6x G.B. Aitken, J.L. Duncan, G.P. McQuillan, J. Chem. Soc. Ž1971. 2695. w7x J.F. Arenas, F. Marquez, A. Cardenete, Spectrochim. Acta 40A Ž1984. 1033. w8x R.M. Badger, R.D. Waldron, J. Chem. Phys. 26 Ž1957. 255. w9x B. Schrader, W. Meier, K. Gottlieb, H. Agatha, H. Barentzen, P. Bleckmann, Ber. Bunsenges 25 Ž1971. 1263. w10x P. Bleckmann, B. Schrader, W. Meier, H. Takahashi, Ber. Bunsenges 75 Ž1971. 1279. w11x A. Vijai, D.N. Sathyanarayana, Spectrochim. Acta 49A Ž1993. 1565. w12x H. King, J. King, J. Raman Spectrosc. 24 Ž1993. 77.
w13x R.J. Meier, B.J. Coussens, J. Mol. Struct. ŽTheochem.. 253 Ž1992. 1565. w14x S. Sanka Bala, P.N. Gosh, J. Mol. Struct. 101 Ž1983. 69. w15x M.J. Frisch, G.W. Trucks, H.B. Sclegel, P.M.V. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, A. Al-Laham, V.G. Zakrewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen., M.W. Wong, J.L. Andres, E.S. Replogie, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzales, J.A. Pople, GAUSSIAN 94, Gaussian, Pittsburgh, PA, 1995. w16x A.D. Becke, Phys. Rev. B 38 Ž1988. 3098. w17x C. Lee, W. Yang, R.G. Parr, Phys Rev. A 37 Ž1988. 785. w18x S.H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 58 Ž1980. 1200. w19x R. Poirier, R. Kari, I.G. Csizmadia, Handbook of Gaussian Basis Sets. A Compendium for ab initio Molecular Orbital Calculations. Elsevier, Amsterdam, 1985, and refs. therein. w20x P.D. Godfrey, R.D. Brown, A.N. Hunter, J. Mol. Struct. 413 Ž1997. 405. w21x M. Spoliti, A. Pieretti, L. Bencivenni, N. Sanna, Electron. J. Theor. Chem. 2 Ž1997. 1. w22x D. Ferro, R. Martino, G. Della Gatta, J. Chem. Thermodyn. 26 Ž1984. 183. w23x S.T. King, Spectrochim. Acta 28A Ž1972. 165. w24x S. Suzer, L. Andrews, J. Chem. Phys. 87 Ž1987. 5131. w25x J.R. Durig, D.W. Wertz, J. Chem. Phys. 46 Ž1967. 3069.
Matrix and ab initio infrared spectra of thiourea and thiourea-d 4 L. Bencivenni
a,b
, S. Nunziante Cesaro
c,)
, A. Pieretti
a,b
a
c
Dip. Chimica, UniÕersita’ ‘La Sapienza’, 00185, Rome, Italy b Istituto Nazionale di Fisica della Materia (INFM), Italy C.S. per la Termodinamica Chimica alle Alte temperature, c r o Dip. Chimica, UniÕersita’ ‘La Sapienza’, P.le A. Moro, 5, 00185, Rome, Italy Received 15 April 1998; revised 12 September 1998; accepted 16 September 1998
Abstract The FTIR spectrum of thiourea and thiourea-d 4 isolated in argon and nitrogen matrices are presented for the first time and discussed in terms of normal modes predicted by ab initio calculations. Vibrational frequencies, IR intensities and isotopic shifts obtained from the density functional B3-LYPr6-31q GŽ2d,p. calculations show the most satisfactory agreement with experiments. In contrast with previous results, the present theoretical study indicates non-planarity of the molecule. The lowest energy structure has C 2 symmetry and the less stable isomers has C s symmetry. Experimental data clearly indicate the presence of a single isomer in the vapor. The decomposition of thiourea for vaporization temperature over 410 K is also discussed. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Vaporization temperature; IR intensities; Isotopic shifts; Thiourea-d 4 ; Vibrational frequencies
1. Introduction The vibrational spectrum of thiourea has continued to be studied since the first paper by Edsall in 1937 w1x. The first tentative assignment of thiourea molecule in alkali halide pellets was reported by Stewart w2x. The assignment for in plane vibrations has been supported by the normal coordinate treatments of thiourea and thiourea-d 4 by Yamaguchi et al. w3x and Hadzi et al. w4x who investigated also the infrared spectra of thiourea in nujol and CH 3 CN
) Corresponding author. Tel.: q39-6-49913962; Fax: q39-649913951; E-mail: [email protected]
solution and included in the work hydrogen and nitrogen isotopically substituted analogues. Further support came from the work of Peyronel et al. w5x on thiourea dry powder spread over KBr pellets. In plane and out of plane fundamentals of thiourea have been calculated using a general force field by Aitken et al. w6x and Arenas et al. w7x, respectively. Infrared polarized spectra of single crystals of thiourea have been analyzed long ago in the N–H stretching region only w8x. A complete analysis of single crystal of thiourea has been given by Schrader et al. w9x and Bleckmann et al. w10x both in infrared and Raman. Quite recently, spectra, geometry and force field of thiourea and its deuterated and 15 N isotopomers have been the object of an ab initio extensive study w11x. In all the aforementioned works, bands are assigned assuming a C 2v symmetry for the thiourea molecule.
0924-2031r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 3 9 - 3
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L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
It is worth noticing that thiourea adsorbed on roughened silver has recently attracted renewed interest because of its surface enhanced Raman scattering ŽSERS. behavior depending on complexation, protonation and co-adsorption w12x. In this context, the interaction between the molecule and silver atoms present some interest. Therefore, we have undertaken a systematic investigation on the formation of the silver–thiourea complex using the matrix isolation technique. Since spectroscopic data on the free molecule are lacking in the literature, the first step of the work necessarily involved the complete characterization of the isolated thiourea. In this work spectra of argon and nitrogen isolated thiourea and thiourea-d 4 are therefore presented and discussed in terms of potential energy distribution analysis predicted by ab initio calculations.
2. Experimental The experimental apparatus basically consists of a cryotip ŽAir Products CSA202. connected under rotary vacuum to a Bruker Interferometer IFS 113 v through a suitable IR transparent window ŽCsI or polyethylene.. The gold plated copper cold finger is free to rotate in a homemade shroud under high vacuum Ž10y6 Torr.. The isolating gases Žargon or nitrogen of high purity supplied by Caracciolo. reached the cold finger through a stainless steel inlet equipped with a standardized needle valve. A flow rate of 1 mmolrh was needed for a good isolation degree. Thiourea ŽICN Biochemicals, 99%. and thiourea-d 4 Ž98% enriched, Aldrich. were vaporized in the 350 to 390 K temperature range Ž10y7 –10y6 atm.. Over 410 K, the molecule partly decomposes in ammonia and isothiocyanic acid. Deposition times lasted from 15 to 90 min. Spectra were taken in reflection, accumulating at least 200 scans at a resolution of 1 cmy1 or better. Annealing cycles were normally performed rising the temperature of the matrix up to 30 K and recooling.
3. Vibrational assignment The FTIR spectra of argon and nitrogen thiourea and deuterated analogues, obtained under identical experimental conditions, looked always very similar showing only frequency shifts due to the different environment. Therefore, in Figs. 1 and 2 are reported only spectra of argon isolated molecules, including features observed under 200 cmy1 where nitrogen looses its transparency. Experimental frequency values of fundamental modes of thiourea and thiourea-d 4 are summarized in the Tables 1 and 2. Only the internal coordinates having a strong contribution in each mode appear in the assignment while the complete picture is reported in Section 4.2 ŽTable 3.. The assignment is proposed in terms of a non-planar C 2 symmetry according to the calculations reported in Section 3.1. So far, this finding is in contrast with the assumption of planar geometry universally adopted in the literature but agrees with recent ab initio results on urea molecule w13x. For sake of simplicity the observed peaks and assignment are discussed separately for each spectral region. 3.1. N–H stretching modes In the upper frequency region ŽFig. 1a. the N–H stretching modes are expected. In argon isolated thiourea, in fact, four peaks have been detected: a strong absorption is shown at 3544.9 cmy1 , a medium intensity doublet at 3431.6r3423.0 cmy1 and two weak bands at 3682.0 cmy1 and 3494.9 cmy1 . Corresponding features in nitrogen appear at 3540.8, 3429.2r3420.3, 3663.5 and 3493.1 cmy1 , with approximately the same intensity ratio. In both media, the intensity of the features observed is unaffected upon annealing cycles. The excellent agreement between calculated and observed intensities confidently suggests to assign the bands at 3682.0 and 3494.9 cmy1 to the symmetric stretching modes of A type and peaks at 3544.9 and 3431.6r3423.0 cmy1 to the B type asymmetric stretchings Žsee Table 1.. Since combination bands and overtones are not expected in this spectral region, the splitting of the asymmetric
Fig. 1. The FTIR spectrum of argon isolated thiourea Ža., N–H stretching region Žb., NH 2 bending and C–N stretching region Žc., NH 2 rocking and C5S stretching region Žd., low frequency mode.
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
93
94
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
Fig. 2. The FTIR spectrum of argon isolated thiourea-d 4 Ža., N–D stretching region Žb., ND 2 bending and C–N stretching region Žc., ND 2 rocking and C5S stretching region Žd., low frequency modes.
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102 Table 1 IR experimental frequencies Žcmy1 . of argon and nitrogen isolated thiourea Thiourea in Ar
Thiourea in N2
Assignment a
3682.0 3544.9 3494.9 3431.6r3423.0 1613.8 1592.9 1412.7 1391.4 1053.8 1035.9 758.9 307.5 291.5 281.5
3663.5 3540.8 3493.1 3429.2r3420.3 1617.7 1594.9 1419.3 1393.4 1059.4 1034.4 753.8
ns N–H ŽA. nas N–H ŽB. ns N–H ŽA. nas N–H ŽB. d NH 2 ŽA. d NH 2 ŽB. nas C–N ŽB. ns C–N ŽA. r NH 2 ŽB. r NH 2 ŽA. n C5S ŽA. d C5S ŽB. v NH 2 ŽB. t NH 2 ŽA.
a
Fundamental modes are classified according to C 2 group of symmetry Žsee text.. n:stretching, d:bending, r:rocking, v:wagging, t:torsion. Symmetry characters of the modes are given in parentheses.
stretching Ž3431.6r3423.0 cmy1 in argon and 3429.2r3420.3 cmy1 in nitrogen. in both the environment still remains unexplained. In the same spectral region, water traces are readily identified at 3731.3 and 3642.0 cmy1 in argon and 3726.4 and 3634.5 cmy1 in nitrogen, respectively. It is worth observing that frequency values of N–H stretching modes of free thiourea are significantly higher than those of the corresponding modes of solid thiourea Ž3390–3090 cmy1 . w1,2,4,8–10x and those of thiourea in CH 3 CN solution Ž3473–3358 cmy1 . w4x. The continuous blue shift in the three states Žsolid, solution and free molecule. is attributed to the weakening of the hydrogen bond effect, passing from solid to solution states, until the complete loss of coupling in the isolated molecule. Literature data concerning isotopic derivatives of thiourea are quite poor. Hadzi et al. w4x reported the N–D stretching modes of thiourea-d 4 in solid and solution states in the range 2540–2360 cmy1 and 2605–2444 cmy1 , respectively. In the spectral region where N–D stretching modes are expected, argon isolated thiourea-d 4 showed four bands at 2621.5, 2500.5, 2491.2 and 2464.5 cmy1 . The corresponding features in nitrogen appeared at 2629.8, 2503.2, 2494.2 and 2472.9 cmy1 . N–D stretching
95
modes appear sensibly weaker than corresponding N–H modes, and show a slight different intensity ratio. For this reason, the correspondence proposed in the table, grounded only on the calculated values, must be considered tentative. 3.2. NH2 bending modes (d NH2 ) Symmetric and asymmetric bending modes Ž d NH 2 . of thiourea coincide in a single broad band, lying in the tight interval 1617–1590 cmy1 , both in solid w2,4,14x and solution states w4x. Only dry powdered thiourea samples spread over KBr pellets w5x showed two distinct peaks at 1612 and 1590 cmy1 . The complete deuteration of the molecule, increased the frequency difference between the two modes allowing the observation of both of them at 1189 and 1141 cmy1 in solid w4x and 1199 and 1140 cmy1 in CH 3 CN solution w4x. Normal coordinate analysis, available in the literature, appears quite contradictory. Yamaguchi, in fact, predicted A and B type d NH 2 at 1623 and 1619 cmy1 , respectively, and the corresponding d ND 2 at 1204 and 1188 cmy1 , in close agreement with experimental data w3x. In contrast, Aitken et al. calculated symmetric and asym-
Table 2 IR experimental frequencies Žcmy1 . of argon and nitrogen isolated thiourea-d 4 Thiourea-d 4 in Ar
Thiourea-d 4 in N2
Assignment a
2621.5 2500.5 2491.2 2464.5 1431.0 1353.9 1276.7 1127.4 879.4 811.9 710.1 255.9 229.4 211.2
2629.8 2503.2 2494.2 2472.9 1449.3 1355.8 1284.2 1131.7
n s N–D ŽA. n as N–D ŽB. ns N–D ŽA. nas N–D ŽB. nas C–N ŽB. ns C–N ŽA. d ND 2 ŽA. d ND 2 ŽB. r ND 2 ŽA. r ND 2 ŽB. n C5S ŽA. d C5S ŽB. v ND 2 ŽB. t ND 2 ŽA.
a
Fundamental modes are classified according to C 2 group of symmetry Žsee text.. n:stretching, d:bending, r:rocking, v:wagging, g:out of plane, t: torsion. Symmetry characters of the modes are given in parentheses.
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L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
metric d NH 2 at 1634 and 1625 cmy1 but sensibly higher frequency values for corresponding d ND 2 Table 3 B3-LYPr6-31qGŽ2d, p. harmonic frequencies of vibration Žcmy1 . and potential energy distribution of the most stable C 2 symmetry isomers of thiourea and thiourea-d 4 Vibrational frequencies Žcmy1 . a Thiourea 3702 Ž26. 3701 Ž68. 3571 Ž13. 3563 Ž48. 1658 Ž74. 1632 Ž211. 1427 Ž315. 1411 Ž315. 1072 Ž19. 1068 Ž73. 766 Ž12. 636 Ž4. 580 Ž91. 497 Ž40. 458 8Ž4. 401 Ž4. 386 Ž316. 337 Ž128. Thiourea-d 4 2749 Ž21. 2737 Ž32. 2581 Ž16. 2574 Ž52. 1445 Ž277. 1376 Ž428. 1202 Ž29. 1157 Ž12. 895 Ž17. 834 Ž5. 713 Ž10. 630 Ž6. 416 Ž47. 401 Ž13. 379 Ž19. 367 Ž10. 292 Ž162. 244 Ž64. a b
Potential energy distributionb
ŽA. ns N–H Ž96. ŽB. nas N–H Ž96. ŽA. ns N–H Ž95. ŽB. nas N–H Ž96. ŽA. d NH 2 Ž92. ŽB. d NH 2 Ž87.q nas C–NŽ12. ŽB. nas C–N Ž70.qr NH 2 Ž12.q n C5S Ž10. ŽA. r NH 2 Ž37.q ns C–N Ž24.q n C5S Ž20.q v NH 2 Ž16. ŽB. r NH 2 Ž84.q nas C–N Ž15. ŽA. ns C–N Ž53.qr NH 2 Ž35.q n C5S Ž11. ŽA. n C5S Ž66.q ns C–N Ž17. ŽB. g C5S Ž91. ŽB. t NH 2 Ž81.q v NH 2 Ž16. ŽA. t NH 2 Ž42.q v NH 2 Ž38.qd NCN Ž10. ŽA. d NCN Ž76.qr NH 2 Ž13. ŽB. d C5S Ž91. ŽB. v NH 2 Ž62.q t NH 2 Ž24.q ns C–N Ž13. ŽA. t NH 2 Ž42.q v NH 2 Ž38.qd NCN Ž10.
ŽA. ns N–D Ž97. ŽB. nas N–D Ž98. ŽA. ns N–D Ž97. ŽB. nas N–D Ž97. ŽB. nas C–N Ž77.qd ND 2 Ž11. ŽA. ns C–N Ž44.q n C5S Ž19.qd ND 2 Ž15.q d NCN Ž12.qrND 2 Ž10. ŽA. d ND 2 Ž67.q n C5S Ž12.qr ND 2 Ž11.q d NCN Ž10. ŽB. d ND 2 Ž88. ŽA. ns C–N Ž44.qr ND 2 Ž41.qd ND 2 Ž10. ŽB. r ND 2 Ž85. ŽA. n C5S Ž68.qr ND 2 Ž15.q ns C–N Ž13. ŽB. g C5S Ž87. ŽB. t ND 2 Ž47.q v ND 2 Ž16. ŽB. d NCN Ž58.q v ND 2 Ž27.qr ND 2 Ž15. ŽA. v ND 2 Ž46.qd NCN2 Ž20.q t ND 2 Ž17.q r ND 2 Ž10. ŽB. d C5S Ž79.qr ND 2 Ž11. ŽB. v ND 2 Ž61.q t ND 2 Ž27.q nas C–N Ž11. ŽA. t ND 2 Ž49.q v ND 2 Ž35.
IR intensities ŽKm moly1 . are reported in parentheses. P.E.D. contribution lower than 10% are not reported.
Ž1388 and 1507 cmy1 . w6x. A more recent ab initio investigation placed the asymmetric d NH 2 at 1622 cmy1 Ž1159 cmy1 for d ND 2 . and the symmetric one at 1657 cmy1 Ž1200 cmy1 for d ND 2 . with approximate intensity ratio 3:1 w11x. Our theoretical analysis predicts the B and A type d NH 2 at 1632 and 1658 cmy1 with intensity ratio 3:1. The corresponding vibrations of thioures-d 4 are calculated at 1157 and 1202 cmy1 , respectively. According to our theoretical treatment both bending modes of thiourea-d 4 should be much less intense than corresponding modes of thiourea. In addition, the symmetric bend of thiourea-d 4 is expected twice more intense than the asymmetric one, with a trend opposite to that shown by the corresponding vibrations of thiourea. Two intense features, at 1613.8 and 1592.9 cmy1 in argon and 1617.7 and 1594.9 cmy1 in nitrogen, have been assigned to the symmetric and asymmetric d NH 2 ŽFig. 1b., since they match the required spectral position and intensity ratio inferred from literature data and our theoretical results. In both the environment, the two fundamental modes embed the bending mode of water Ž1599.6 and 1597.9 cmy1 in argon and nitrogen, respectively. which could not be eliminated from the vaporization of the sample. Corresponding modes of argon isolated thiourea-d 4 have been individuated at 1276.6 and 1127.4 cmy1 ŽFig. 2b. for two main reasons: their closeness to experimental frequency values of d ND 2 in solid and solution states w4x and to values calculated by ab initio treatment Žw11x and this work.. In addition, the intensity ratio of the mentioned bands mirrors quite well the ratio predicted in this work Žsee Table 3.. In nitrogen environment the d ND 2 are detected at 1284.2 and 1131.7 cmy1 and appear weaker than in argon, as observed for the N–H stretching modes, but still keep the predicted intensity ratio. 3.3. The 1400–1000 cm y 1 region In the 1400–1000 cmy1 spectral region four fundamental modes are expected and are due to different contributions of C–N stretching, NH 2 rocking Žr NH 2 . and C5S stretching in minor extent. Literature data show a substantial agreement in assigning two intense peaks around 1400 cmy1 to the C–N stretching mode and two bands of medium
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
intensity around 1000 cmy1 to the r NH 2 w2,4,6x. However, in the study of dry powdered thiourea the rocking modes were placed at higher frequency values Ž1210 and 1210 cmy1 . w5x. The attribution of the C–N stretching modes of thiourea-d 4 was not settled in early literature and the frequency value of these bands ranged from 1400 to 1100 cmy1 w4,6x. The rocking modes of the ND 2 group were however individuated in the 900–800 cmy1 interval w4x. Frequency values and intensity of bands having a strong C–N contribution resulting from the theoretical treatment presented in this work ŽTable 3. match fairly well previous ab initio results w11x. On the other hand, the assignment of the NH 2 rocking modes shows some discrepancy. In fact, the asymmetric r NH2 is located at the high frequency side of the symmetric one Ž1072 and 1068 cmy1 , respectively. in contrast with the previous evaluation Žasymmetric r NH 2 at 1051 cmy1 and symmetric r NH 2 at 1055 cmy1 . w11x. In argon isolated spectra of thiourea, two bands are shown at 1412.7 and 1391.4 cmy1 Ž1419.1 and 1393.4 cmy1 in nitrogen. ŽFig. 1b.. The spectral position and the intensity ratio of these peaks strongly suggest their assignment to modes heavily involving the C–N stretching of B and A type, respectively. Upon complete deuteration the corresponding features are individuated at 1431.0 and 1353.9 cmy1 in argon and 1449.3 and 1355.8 in cmy1 nitrogen. The increase of the spread between them and the decrease of their intensity ratio are in excellent agreement with theoretical predictions Žthis work and Ref. w11x., thus giving further support to the attribution. In matrix isolated thiourea spectra, bands attributable to modes with strong contribution of the NH 2 rocking are observed at 1053.8 and 1035.9 cmy1 in argon ŽFig. 1c. and 1059.4 and 1034.4 cmy1 in nitrogen ŽTable 1.. In argon, both bands have approximately the same intensity while in nitrogen the high frequency peak appears of double intensity with respect to the remaining one. The marked sensitivity to the environment is not surprising on consideration of the specific interaction of the N–H bond toward the N2 molecule. The same explanation could apply to the absence of corresponding modes in nitrogen isolated thiourea-d 4 . In argon, however, both modes are clearly detected at 879.4 and 811.9
97
cmy1 ŽFig. 2c. having approximately the intensity ratio predicted in the theoretical treatment presented here Žsee Section 4.2 and Table 3.. 3.4. The 1000–200 cm y 1 region In this region the C5S stretching and the C5S out of plane bending modes are expected in the 800–600 cmy1 interval, along with six further fundamentals under 600 cmy1 containing the participation of the C5S and N–C–N bending and the torsion Ž t NH 2 . and wagging modes of the NH 2 group Ž v NH 2 .. A band lying around 730 cmy1 observed in spectra of thiourea in solid, solution or dry powder samples has been assigned to the C5S stretching w4,5x with the exception of Stewart who attributed an intense peak appearing at 730 cmy1 in solid thiourea spectra to the C–N stretching w2x. In totally deuterated samples the band shifted about 50 toward the red w4x. Normal coordinate analysis w6x and former ab initio w11x calculation support the mentioned frequency values. The theoretical treatment presented here provides 766 and 713 cmy1 for the C5S stretching of thiourea and thiourea-d 4 , respectively, in good agreement with the experimental values of argon isolated thiourea and thiourea-d 4 at 758.9 and 710.1 cmy1 , respectively ŽFig. 1cFig. 2c.. In nitrogen, the corresponding feature of thiourea was observed at 753.8 cmy1 but, unfortunately, the C5S stretching of thiourea-d 4 was not detected even in concentrated spectra. The individuation of modes lying under 600 cmy1 in literature data is far from exhaustive: an intense peak detected at 486 cmy1 by several authors in solid thiourea samples w2,4,5x has been mainly assigned to the N–C–N bend and a medium intensity band at 411 cmy1 has been attributed to NH 2 torsion w2x or N–C5S bend w4x. The latter hypothesis finds some support in the normal coordinate analysis reported by Aitken et al. w6x. The frequency values resulting from our theoretical treatment are collected in Table 3, together with the proposed assignment. It is however worth noticing that calculated intensities are quite different from those proposed in previous ab initio analysis w11x. In the 600–30 cmy1 region, argon isolated thiourea shows three bands at 307.5, 291.5 and 281.5 cmy1 ŽFig. 1d.. Upon complete deuteration the three
98
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
bands shift at 255.9, 229.4 and 211.2 cmy1 . Corresponding features in nitrogen have not been observed due the loss of intensity of this matrix in the far infrared region. Since the agreement between experimental and theoretical values is quite poor in this spectral range, the assignment proposed in Tables 1 and 2 to fundamental modes of thiourea and thiourea-d 4 , respectively, must be considered only tentative.
4. Computational study The Gaussian 94 package programs w15x was used for running the ab initio and density functional calculations reported in this work. The geometry optimizations of the structures of thiourea were carried out at the second order of the Moller–Plesset theory including all electrons in the correlation calculation and using the B-LYP and B3-LYP density functionals w16–18x. The DZŽd,p. and the 6-31 q GŽ2d,p. basis sets w19x stored in the Gaussian 94 program w15x, were chosen for the study. The scanning of the energy surface of thiourea was performed at the second order of the Moller–Plesset level considering all electrons active with the double-zeta ŽDZ. basis set augmented with multiple polarization functions.
These calculations are indicated in the work as MP2rDZŽd,p., MP2rDZŽ2df,2p. and MP2rDZ Ž3df,2p.. A further stability study was carried out at the fourth order of the Moller–Plesset theory within the frozen-core approximation MP4rDZŽd,p.. The single-point energy computations were performed on the MP2rDZŽd,p. optimized geometry of thiourea. 4.1. Structures In this section of the study we report the main results accomplished from the computations. The Moller–Plesset and the B3-LYP density functional calculations indicate that there are two stable non-planar isomers of the molecule. The lowest energy structure has C 2 symmetry ŽFig. 3a. and the less stable isomer has C s symmetry ŽFig. 3b.. Nonplanarity of these isomers is due to the NH 2 groups of the molecule. On the contrary, the N–CS–N moiety of both the C 2 and C s symmetry structures is planar. For this reason, thiourea and urea w13x are isostructural molecules. The present theoretical study indicates without any doubt non-planarity of these isomers and therefore there is not agreement with the previous theoretical conclusions w11x which attributed to thiourea molecule a planar structure of C 2v symmetry. Such discrepancy is likely due to the
Fig. 3. Non-planar lowest energy structures of thiourea: Ža. C 2 symmetry isomer, Žb. C s symmetry isomer and Žc. C 1 symmetry transition state. Higher energy structures: Žd. planar C 2v symmetry, Že. non-planar C s symmetry and Žf. and Žg. non-planar C 2v symmetry structures.
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
use of the 3-21G) basis sets w11x. The results of the single-point energy calculations are summarized in Table 4 and indicate that the C 2 symmetry isomer is more stable than the C s symmetry one within a few kJ moly1 . In addition to this, the isomerization barrier is quite low so that the conversion of the C s isomer into the C 2 symmetry one is straightforward. From the theoretical data determined for the C 2 and C s symmetry isomers one might in fact conclude that the most stable C 2 isomer is nearly exclusively present in the vapor at the vaporization temperature of the FTIR measurements. This is indeed quite important for the interpretation of the FTIR spectrum of matrix isolated thiourea. The most stable isomers are separated by the transition state shown in Fig. 3c having one planar NH 2 group. The remaining structures of thiourea which were taken into account are lying in the high energy region of the energy surface and are second order saddle pints. Among them, the C 2v symmetry one shown in Fig. 3d has planar structure and is 15 kJ moly1 unstable with respect to the most stable isomer. This is an interesting result because it shows without any doubt that the planar C 2v symmetry structure, indicated elsewhere as the stable equilibrium structure of thiourea w11x, is not the stable minimum of the molecule. The other non-planar models are the C s ŽFig. 3e. and the C 2v ŽFig. 3f and g. symmetry and are lying above 100 kJ moly1 with respect to the lowest energy minimum isomer. Evi-
99
dently these structures, which are all second order saddle points, have no practical interest, being unstable forms. The summary of the calculations is given in Table 4 Žstability. and Table 5 Žgeometries and harmonic vibrational frequencies.. The results of our theoretical study on thiourea molecule suggests a close similarity with quite recent conclusions obtained for urea molecule w20,21x. 4.2. Theoretical Õibrational assignment The theoretical vibrational spectrum of the most stable isomer of thiourea is important for understanding the matrix isolation study of the free molecule. The MP2 and density functional calculations of the C 2 symmetry isomer provide nearly identical IR patterns. However, the best theoretical result was obtained from the density functional B3-LYPr6-31 q GŽ2d,p. calculation which shows the most satisfactory agreement with the experiments. We have also determined the potential energy distribution of this isomer from the MP2 and the density functional frequencies of vibration and both these computational methods provide the same conclusions, that is the same theoretical assignment of the calculated harmonic vibrational frequencies. The potential energy distribution was determined by using a computer program written by one of the authors. This program employs the cartesian force constants supplied from the Gaussian 94 program
Table 4 Relative energies Ž D E, kJ moly1 . calculated with respect to the C 2 symmetry isomer of thiourea MP2rDZ Žd,p.
MP2rDZ Ž2df,2p.
MP2rDZ Ž3df,2p.
MP4rDZ Žd, p.
DZPE a
Stable isomers C2 Cs
0 4
0 3
0 5
0 5
0 y2
Transition state C1
5
3
5
6
y3
8
9
High energy second order saddle point C 2v 6 Cs 126 C 2v 136 C 2v 142 a
Zero-point energy difference calculated at the MP2rDZŽd,p. level.
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Table 5 Optimized equilibrium geometries Ža., harmonic frequencies Žcmy1 . Žb.and IR band intensities Žc. of the C 2 and C s symmetry isomers of thiourea Geometriesa
b
Cs
C2
Cs
C2
C 2 symmetry
MP2r DZŽd,p.
MP2r DZŽd,p.
B-LYPr DXŽd,p.
B-LYPr DXŽd,p.
B-LYPr 6-31qG Ž2d,p.
MP2r DZŽd,p.
B-LYPr DZŽd,p.
B3-LYPr 6-31qG Ž2d,p.
Symmetry
MP2r DZŽd,p.
B-LYPr DZŽd,p.
Symmetry
1.657 1.377 1.008 1.009 123.0 113.8 119.9 115.0 116.3 23.5 169.9 y161.8 y15.4
1.682 1.394 1.020 1.023 123.2 113.6 117.8 117.8 114.9 29.8 168.5 y150.2 y11.5
1.686 1.388 1.018 1.021 5 122.6 114.7 114.95 114.95 116.7 22.1 170.8 y162.4 y13.7
1.6695 1.365 1.008 1.011 122.6 5 114.7 115.5 119.8 5 116.3 23.1 170.3 y156.9 y9.7
3781 3781 3645 3641 1689 1669 1460 1430 1102 1100 799 649 612 587 461 438 401 391
3595 3595 3460 3543 1597 1573 1364 1360 1038 1031 737 610 559 562 Ž91. 435 414 383 366
3702 Ž26. 3701 Ž68. 3571 Ž13. 3563 Ž48. 1658 Ž74. 1632 Ž211. 1411 Ž315. 1427 Ž103. 1072 Ž19. 1068 Ž73. 766 Ž12. 636 Ž4. 497 Ž40. 580 Ž91. 458 Ž4. 401 Ž4. 386 Ž316. 337 Ž128.
A B A B A B A B B A A B A B A B B A
3809 3807 3667 3659 1697 1664 1460 1448 1090 1068 803 622 578 464 438 412 394 171
3622 Ž12. 3620 Ž59. 3479 Ž7. 3467 Ž23. 1600 Ž73. 1564 Ž189. 1361 Ž238. 1372 Ž99. 1025 Ž48. 1005 Ž18. 740 Ž8. 593 Ž4. 553 Ž49. 437 Ž4. 423 Ž59. 362 Ž396. 378 Ž1. 128 Ž49.
A Y A X A Y A X A Y A Y A Y A X A Y A X A X A X A X A Y A X A Y A Y A
Bond lenghts ŽA., interatomic angles Ž8., dihedral angles Ž8.. cmy1 . c IR intensities Žkcal moly1 .. X H is the H atom of the longest N–H bond distance. b
C s symmetry
X
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
C2
C5S 1.652 C–N 1.384 N–H 1.0095 X N–H 1.011 5 N–C–S 123.8 N–C–N 112.4 H–N–C 116.4 X H –N–C 115.5 X H–N–H 113.5 X N–C–N–H 30.9 N–C–N–H 167.3 X S–C–N–H y149.1 S–C–N–H y12.7
a
Vibrational frequencies and IR band intensities c
L. BenciÕenni et al.r Vibrational Spectroscopy 18 (1998) 91–102
frequency calculations, carried out for the optimized geometry and the non-redundant set of internal coordinates. The B3-LYPr6-31q GŽ2d,p. vibrational frequencies, along with the calculated IR band intensity, and the potential energy distribution of C 2 symmetry thiourea and thiourea-d 4 were quite useful for the assignment of the matrix isolated spectra, as widely discussed in Section 4.1. The last observation to be discussed concerns the expected IR spectrum of the less stable isomer of C s symmetry. As it may be seen in Table 5, the vibrational spectrum of this species might be sufficiently different from that of the most stable and predominant isomer having C 2 symmetry and therefore, one might be able to check experimentally the presence of the less stable form. The fact that the matrix isolated spectra show no evidence of the C s symmetry isomer seems to suggest that the vapor phase consists of the most stable C 2 symmetry molecule exclusively.
5. Decomposition products Long-lasting depositions at different vaporization temperature from 350 to 420 K were performed in order to obtain concentrated samples for a safe characterization of weakest bands of thiourea. According to thermodynamic measurements, thiourea does not show appreciable decomposition before the melting point and totally decomposes during the transition phase w22x. On deposition over 410 K, low intensity bands were detected. The attribution of these bands to the thiourea molecule was ruled out due to the different grow rate with the vaporization temperature. Infrared data cannot give quantitative indication on the decomposition extent. However, the percentage of decomposed molecules should not exceed the 10% of the total amount in the detection limit of the thermodynamic measurement. Decomposition products were obviously observed in the matrix isolated infrared spectra owing to the high sensitivity of this technique. Thiourea should produce isothiocyanic acid ŽHNCS. and ammonia, in analogy with urea which
101
gives HNCO and NH 3 w23x. The presence of ammonia was readily proved by the appearance of its most intense features at 3446. and 974.6 cmy1 , in argon matrices, assigned to n4 and n 2 , respectively, in excellent agreement with literature data Žw24x.. Only the strongest peak at 969.7 cmy1 was observed in nitrogen. A very weak band at 3400.7 cmy1 in argon suggested the presence of traces of ammonia dimer w24x. The formation of HNCS was suggested by the appearance of a doublet at 1996.8r1981.9 and 1996.6r1991.6 cmy1 in argon and nitrogen, respectively, which has been assigned to the C–N stretching mode of this molecule on ground of literature data w25x. On vaporization of thiourea-d 4 over 410 K, some DNCS was formed having the C–N stretch at 1938.7 cmy1 , observed only in argon moiety w25x. Remaining fundamental bands of HNCS and DNCS were never detected in our spectra in agreement with the weaker intensity expected w25x. MP2 and density functional computations gave further support to the assignment of the bands observed to the isothiocyanic acid. For the sake of brevity, the computational details will not be discussed here. Shortly, the treatment predicts the N–H, C5N and C5S stretching vibrations at 3565, 1953 and 712 cmy1 , respectively, and indicates a high intensity in the infrared for the C5N stretching mode as actually observed experimentally.
6. Conclusion Comparison of the spectra of thiourea and thiourea-d 4 in solid, solution and pseudo gaseous states clearly shows marked differences in the region of the N–H stretchings, due to the different efficiency of the hydrogen bonding. Spectra of matrix isolated thiourea and the deuterated analogue are in excellent agreement with calculations, suggesting a non-planar C 2 symmetry, in contrast with previous conclusions. Some discrepancy is however present between experimental and theoretical data in the far infrared region. According to the theoretical treatment, the barrier between the lowest energy state of C 2 symmetry and the less stable isomer of C s symmetry should be
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quite low. However, experiments suggest the presence of a single isomer in the vapor.
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