Ind ian Journal o f Chemistry Vo l. 4 1A, September 2002, pp. 1777-1783

Simulation of the infrared spectra of thioacetamide by the extended molecular mechanics method

D N Sath yanarayana* & E Ganeshsrini vas

Department of Inorgani c and Phys ica l Chemi stry, Indian Institute of Sc ience, Bangalore 560 01 2, Ind ia

Received 4 JUlie 2002

The in fra red spectrum of the matri x- isolated species of thioacetamide has been simul ated using the extended mo lecular mechanics method. The equili briu m structure, vibrati onal freque nc ies, dipo le mo ment and infrared absorptio n intensities of thioacetami de have been calculated in good agreeme nt with the ex perime nt. T he vibratio na l freq uencies and in frared absorption intensities for the isotopic mo lecul es IJCH3CSN H2, C H3CS I5NH2 and 13CH3CS ND2 have also been calculated

consiste nt with the ex perime nt. T he infrared spectra o f the matri x isolated spec ies o f N- and C- deuterated isotopomers of thioacctamide, C H)CSND2 and CD3CSN H2 have also been simul ated in sat isfactory agreement with the experimental spectra.

We presented earlier an empiri cal po tenti al model to successfull y simulate the infrared spectra of two simple primary amides, namel y, formamide and acetamide1,2. The calculated equilibrium structure, thermodynaJ'n ic quantities, normal mode frequencies and infrared absorption band intens ities were found to be III good agreement with the experimental measurements. We have now extended the study to thioacetamide, the sulphur analogue of acetamide. Thioacetamide is a polar molecule like acetamide. From thi s laboratori, a study of the vibrational spectra of thi oacetamide and its 13C_, 15N_ and di fferently deuterium labeled molecules using normal coordinate analys is procedure has already been reported. During the same time, Walter and Stag lich4

have measured the Raman spectra of thi oacetamide and its deuterated isotopes and have carried out normal coordinate analysis using a generali zed valence force field. Suzuki5, however, was the first to perfo rm normal coordinate treatment for the in-plane vibrations of thioacetamide.

The potential function employed in the present study calcul ates the vibrational frequencies and equilibrium structures, which can more appropriately be compared with the gas phase measurements6

,7. We have therefore utili zed the recent infrared spectral measurements of thioacetamide fo r the matrix isolated species by Kundsen el ai. 8

• The potenti al function has been parameteri zed to reproduce the vibrati onal frequencies and the qualitati ve features of the infrared absorption intensities observed for the matrix isolated spectrum of thi oacetamide. The in frared spectra of the

matrix isolated species of thioacetamide and its C­and N- deuterium labeled molecules have been simulated. To check the internal consistency of the fo rce field , the calculations have been extended to 2H, 15N and 13C isotopic molecules of thioacetamide.

Computational Details The calcul ations were performed using the same

potential function as that employed fo r acetamide2•

Initially the values of aj (Morse constant), D je (bond

dissociati on energy) and r jO (bond lengths) were transferred from acetamide . The Dj and aj for C=S and C-N bonds were suitably modified based on chemical intllltlOn to reproduce experimental stretching frequenc ies. For reproducing the bending frequencies,

the values of bending fo rce constants, 8 jO and F jj were transferred from acetamide and modi fied to reproduce the experimental data. The interaction force constants of acetamide have been used fo r thioacetamide without any modificati on except for two new interaction force constants included now between tors ional and out-of-plane coordinates . The tors ional parameters rep roduce well the experimental frequencies. The electronegati vi ty values for H, C, N and S were taken from Pauling9

. The electronegati vity of sulphur was slightly modified, to 2.6, to reproduce the experimental dipo le moments. The fin al potential parameters deri ved for thioacetamide are g iven in Table 1.

The bond charge parameters and charge flu xes fo r the stretching and bending modes were taken fro m acetamide2

. T he values for thioacetamide were

1778 INDIAN 1 CHEM, SEC A, SEPTEMBER 2002

Table I--Potential parameters for thioacetamide

Non-bonded interactions Bending force constants, Fjj rjj (/jj' ajA I3 jj, (/1 Cjj ' A Angle 8j 8jO , deg Fjj, aJ rad-2

S .... S 20.866 389.990 2.980 Bond stretching H-C-H 111.0 0.535 op bending rj aj, A- I Dj, aJ rj , A C-N-H 116.5 0.500 NH2 0.0

0.0030 H-C 1.835 0.628 1.150 N-C=S 121.0 1.563 C=S 0.0

0.4100 C-C 1.880 0.430 1.500 C-C-N 119.0 0.480 C=S 1.800 0.917 1.196 C-C=S 120.0 0.720 C-N 2.000 0.820 1.373 H-C-C 109.5 0.620 N-H 2.300 0.560 1.011 H-N-H 127.0 0.235

Interaction force constants, Fjj 10-8 N rad·1 Stretch-stretch N em-I Stretch-bend Bend-bend, aJ rad-2 Torsion, aJ

C- H, C- H 0.040 H - C -C, 0.140 H -C- C, -0.010 V2j (C-C-I\'-H) -0.0204 C-C H-C-C

C =S,C-C 0.414 C -C = S, 0.252 H -C-C, -0.109 V2j (H-N-C=,S) -0.0204 C=S N-C=S

C-C, C- N 0.350 C-C- N, 0.243 C-C = S, -0.200 V3j (H-C-C-N) -0.0011 C-N N -C =S

C= S,C-N 1.188 C-C-N, 0.048 C -C = S, -0.360 C-C C-C-N

N - H, N - H -0.077 N -C = S, 0.356 C-C- N, -0.337 C=S N-C=S

C - H, N - H 0.030 N -C = S, 0.362 C - C - N, 0.015 C-N C-N-H C - N - H, 0.125 N -C = S, -0.027 C-N C-N-H C-N -H, 0.100 C- N - H, -0. 120 N - N C-N-H H - N - H, 0.049 C - N - H, -0.128 N-H H-N-H

C-C-N-H -0.074 C* (C, N, = S) C-C- N - H, 0.088 N* (Hz, C =)

* Atom going out of the plane formed by the three atoms given in parenthesis.

parameterized to reproduce i :-1C experimental dipole moments and infrared absorption intensities and they are given in Table 2. The bond charge parameters of C = Sand N-H are different from those of acetamide. On the basis of the stretching flux analysis of Gussoni et al. lO

, it may be said that the values for the CN and NH bonds of thioacetamide reflect the polarity of these bonds. The CN bond is more rigid in thioacetamide relative to that in acetamide consistent with a higher torsional barrier about the C-N bondS. The C = S bond is less polar when compared to that of the CO bond and the value for the stretching flux reveals this feature. The bending charge fluxes for the NCS moiety of thioacetamide was modified from that

of the NCO group. For the CNH group, only a sli ght modification was required.

The infrared spectrum was simu lated assuming Lorentzian line shape and standard half bandwidth «(Do) for a known concentration in the range 1800 to 200 cm- ' .

Results and Discussion The calculated and experimental structural

parameters, dipole moment and atomic charges are g iven in Table 3. There is good agreement between the calculated and experimental equilibrium structure". However, the difference between the observed and calculated dipole moment is somewhat

SATHYANARA YANA et at.: SIMULATION OF INFRARED SPECTRA OF THIOACETAMIDE 1779

larger. As is generally the case, the net atomic charges calculated in the present work are overestimates.

Under the point group symmetry Cs, the normal vibrations of thioacetamide are classified as 14A' + 7 A" modes. The observed and calculated vibrational frequencies and infrared absorption intensities for thioacetamide are presented in Table 4. The agreement between the calculated and experimental frequencies is good except for the CH 3 deformations, which show slightly larger deviations. The calculated values for the asymmetric and symmetric CH3

bending modes are 1411 and 1394 cm- I respectively while the experimental values are 1440 and 1369 cm- I

respectively . The assignments for A' and A" vibrations are in

complete agreement with Knudsen et at. 8 excepting

Table 2-Bond charge parameters and bond charge fluxes for thioacetamide

Bond charge parameters, ~, e Bond stretching fluxes, e A- I

H -C 0.1300 8qs ; c / 8Rc; s 0.300

C=S 0.7680 8qCN /8RcN -1.100

C-N 0.1100 8qcc / 8Rcc -0.020

N - H 0.7500 8qNH / 8RNII -0.320

Homogenous y 8qCH /8Rcll -0.100

C-C 0.350 8qcII /8R'cH -0.035

Angle bending fluxes, e rad·1

N - C = S group C - N - H group

8qc ; s / 80s; C - N -0.040 8qCN /80cNH 0.085

8qcs / 80s; C - C 0.040 8qNH / 80cNil 0.065

8qcs / 80c - c - N 0.000 8qCN / 80' HNII -0.170

8qNII / 8011 NH -0. 130

for the NH2 torsion. They have assigned NH2 torsion to a weak band at 592 cm - I . However, the present results suggest that a medium intensity band at 774 cm- I could be assigned to this mode. It is compatible with the NH2 torsional mode assigned at 610 and 622 cm- I respectively in formamide and acetamide\.2 where the CN torsional barrier is relatively lower. Further the calculated infrared absorption intensity apparently supports this assignment. A comparison of the present assignments with the previous work3 from this laboratory shows that there is satisfactory agreement between the two assignments except in the case of two normal modes. The frequencies calculated at 1390 and 1361 cm- I are assigned as C-N stretching and asymmetric CH3 deformation in the previous work and the present work suggests that these assignments have to be reversed.

Illfrared absorption intensities In Table 4 , the calculated infrared absorption

intensities are compared with the observed intensities qualitatively. There is good agreement between the calculated intensities and qualitative features of the infrared band intensity as observed for the matrix isolated spectrum of thioacetamide excepting for the 774 cm·- I band which is calculated as an intense band but experimentally this band is noted to be of medium intensity. The present potential parameters reproduce well the observed infrared absorption intensity for the 370 cm- I band. The low frequency bands are found to be very intense in spectrum of formamide and acetamide and the same trend is found for thioacetamide. Using the calculated vibrational frequencies and their intensities, the infrared spectrum

Table 3--Experimental and calculated geometry and dipole moment and calculated atomic charges for thioacetamide

Bond length, A Bond angle, 0

Bond Experimental" Calculated Angle Experimental Calculated

C - H 1.090 1. 133 L CCS 120.8 120.5 C-C 1.504 1.478 L CCN 116.5 117.0 C=S 1.686 1.647 LNCS 122.7 122.5 C-N 1.318 1.367 LCNHc 121.0 120.8 N - H 1.026 0.994 LCNH, 120.6 120.4

L HNH 11 8.4 11 8.8 Experimental b Calculated

)l, D 4.77 4.43 Net atomic charges C - 0.5150 S - 0.0901 H 0.0548 N - 1.4926 C (S) 0.525R H 0.7037 " From ref. [ IIJ h From ref. r 12]

1780 INDI AN J C HEM , SEC A, SEPTEMBER 2002

Table 4--Ex perimental and calculated vibrati onal frequencies and infrared intensities for thioaeetamicle

Freguenc}' , cm-I

EXEtl. So li d" Matri xh

A' 3295 3520 3080 3404 2945 2963

2850 1650 1596 1479 1440

1390 1369 1362 1346

1303 13 I I

1025 999

972 938 719 728 471 470

377 430 A"· 2977 2920

1440

11 30 101 8

760 774

709 508 51 2 370

" From ref. [3]; b From re f. [8].

'" u c a

%: ' E 1--- ____

VI e ]-

1700

Ca lcd.

3530 3387 294 1 2866 1597 1411 1394 1354 1299 1008 93 1 71 8 471 419 2938 1410 1024 775 501 386 147

( b)

200

Fi g. l--{a) Experimental spectrum of thi oacetamide taken from ref. [8 1 and (b) simulated spectrum , conc. = 0.005 mol L-

I, OJ" =

5.0 cm-I

Exptl."

ms ms w vw

s w m vs m w w w w

w

w w m w

., u c a

%: 'E

VI c a ~

1700

Int. km ma r l

Calcd .

275.6 180.9 0.9 4.2 743.0 15 .3 30. 1 263 .4 254.4 12.6 2 1.6 2.6 18.2 1.6 0.5 3.3 88.6 444.3 7.8 595.0 0.4

Assignment

v"NH2 vsNH 2

v"CH, vsCH, ON H2 O"CH, OsC H, + o"CH, vCN pNH 2 + vCN + vCS pC H, pCH3 + vCS + vCC vCS + vCC oNCS pNCS

v"C H3

O"CH3

pC H3

tCN + nNH2 nCS nNH2 TCH)

( a )

n I '

( b )

1100 800 soc 200 Wavenu mber !em - 1)

Fig. 2--{a) Experimenta l spec trum of CD, CH NH2 taken from re f. [81 and (b) simul ated spec trum, co nc. = 0.003 mol L-I

, OJo = 2.0 · 1 cm

SATHYANARA YANA el al.: SIM ULATION OF INFRARED SPECTRA OF THIOACETAMIDE 178 1

Table 5-Experilllen tal and ca lcu lated vibrational frequencies and infrared intensities for deuterated th ioacetaillide

Frequency, cm-' Exptl. Calcd.

So lid" Matri x"

AN

AN

2945

2485 2285 1486

1360 1240 11 64 979 836 694 420 374 298 1 1424

580 523

3296 3077

1653 1393 1303 1094

957

663 453 343

1038 812 709

" Frolll ref. 13, 81

2631 2477 1447 1388 1363 1257 11 40 983 816 698 42 1 37 1

1447

668 510 275

352 1 3404

1597 1358 1322

1034 926

677

402

1046

77 1 450 370

294 1 2866 26 19 2454 1419 1395 1369 12 16 11 41 947 804 702 446 406 2938 1408 10 18 635 453 287 146

3530 3387 2195 207 1 1596 1352 1300 1093 1010 950 775 665 453 382 2 191 10 11 876 723

468 385 106

o f the matri x i so lated spec ies8 of thioacetamide has

been simulated in good agreement with experimental

spectrum in the region 1800 to 200 cm- 1 as shown in Fig. I.

Labeled molecules To check the intern al consistency of the force

f ield, the vibrational frequencies for the C- and

Int. kmlllo)"' Expt l." Ca lcd.

W

Will Will vs III

III

W

Will W

W

W

vw

Will

W

vw

W

illS illS

vs

illS

W

W

W

vw

W

vw

VS

1.1 2.9 157.1 192.5 197.6 49.3 581.0 132. 1 34.5 6.5 25.8 3.5 14_3 5.6 0.5 3.5 46.4 265.7 57.7 288.5 0.6

257.5 181.3 5. 1 1.6 740.6 310.2 296.4 1.2 4. 1 29.4 3.4 3.0 16.5 1.0 2.5 6.0 286.2 247.9 0.2 595.8 0.7

Assignillent

v"CH, v,CH, v"ND2 v,ND2 b"CH, + b,CH, b"CH, vCN + b,CH, vCS + p D2 + vC + bNCS bND2 pCH, pND2 vCS +vCC bNCS pNCS v"CH3

b"CH] pCH, nCS + nND2 nCS TCN + TND2 TCC

v"N H2 vsNH2 v"CD, v,CD) bN H2 vCN pNH2 + vCN + vCS bsCD, b"CD} vCS + pNH2 pCD) vCS + pCD, bNCS pNCS v"CD, b"CD, pCD3

pCD} + TCN + TNH2 nCS nNH2 TCH,

N-deuterated isotopomers namely CD}CSNH2 and

CH}CSND2 have been calculated and compared w ith

the experimental datas.8 in Table 5 and the agreement

between experimental and calcu lated vibrational

frequencies IS sati sfactory. Regarding the infrared

absorption intensiti es, the agreement IS sati sfactory

between the ca lculated and qualitati ve features of the

1782 INDIAN J C HEM, SEC A, SEPTEMB ER 2002

Table 6--Ex perimental frequenci es (c m· l) and observed and ca lcul ated shifts for selec ted bands o f I:lC and 15N isotopic

mo lec ules of thi oacetamide"

C H,CS I5NH2 IJCH)CSN H2 I:lC H)CSND1

Freq uency S hift ~ Freq ue ncy Ex ptl. Obsd. Ca lcd. Exptl.

1640 -8 -8 1648 1475 - 4 0 1478 1387 -3 0 1389 136 1 - I - 7 135 1 1300 -3 - 5 1303 1024 - I -2 1022 970 -2 - 2 965 714 -5 - 3 710 704 -5 -6 703 512 0 0 512 466 -5 - 5 470

" From ref. [4, 12 J; b Sh i ft !'J. v = V isolOpomcr - V p"rcnt Illoleeule

~ u C Cl

E VI C o '-t-

1700

(a)

( b )

Wavenumber Icm- 1)

Obsd.

0 -I - I

- II 0 - 3 -7 - 9 -6 0

- I

200

Fig. Ha) Experimental spectrum o f C H)CSND2 taken from ref. [8] and (b) s imul ated spect rum, conc. = 0.002 mo l L- I, Wo = S.O

· 1 cm

Shift Frequency Shift Ca lcd. Exptl. Obsd. Ca lcd.

0 1506 0 -2 1485 - I - 2 - 7 142 1 -3 -6 -3 1367 - 7 -7 - I 1236 - 4 - 2 -8 - 8 - 5 973 - 6 - 5 0 83 1 -5 - 3

- 7 693 - I - 7 0 523 0 0 -5 478 - 2 - 7

observed band intensities for these two molecules. The spec tra o f the matri x isolated species of CD,CSNH2 and CH}CSND2 are avai b ble. The spec tra of these two 2H labeled mo lecu les have been

simu lated in the region 1800 to 200 cm- I and they are given in Figs 2 and 3 respecti ve ly. There is sati sfactory agreement between the simul ated and ex perimental spectra. The spectrum of the matrix isolated species of CD3CSND2 is not available. It is we ll known that amides and thioamides are strong ly associated in the solid state th rough hydrogen bonding. It results in the shift of the NHz and thioamide group vibrations in the solid state. Therefore the infrared spectrum of C D1CSN D2 was not simul ated.

The vibrational spectra fo r 13C_ and 15N_ isotopic molecu les namely, 13CH3CSNH2, CH:\CS I5NH2 and 13CH3CSND2 have been reported but for co mpounds in the solid state. Therefore o nly the calculated and the observed sh ifts for these isotopic mo lecules are g iven in Table 6 (ref 12). We find that the calcul ated frequen cy shifts are in satisfactory ag reement with experimental shifts.

Conclusions The present poten ti a l and in frared intens ity

parameters reproduce satisfactori Iy the experimental equilibrium geometry , dipo le mo ment and the vibrational spec tra of thioacetamide and its isotopic molecules. The estimated intens ity parameters are consistent with the po lar nature of thioacctamide . The simulated spectra are in sati sfac to ry agreement w ith the experimental spectra.

SATI-IY ANARA Y ANA et al.: SIMULATION OF INFRARED SPECTRA OF TI-IIOACETAMIDE 1783

Acknowledgement The authors are greatly thankful to Prof. K Machida

and Dr. Y Miwa (Kyoto University, Japan) for their help by providing the RISE program.

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Y, J lIIolec Struct (Th eochelll ), 36 1 (1 996) 2 17. 3 Ray A & Sathyanarayana D N, Bull chelll Soc Japan , 47

(1974) 729. 4 Walter W & Staglich P, Spectrochilll Acta, 3 (1974) 1739.

5 Suzuki I, Bull chem Soc Japan , 35 (1962) 1449.

6 Machida K, Noma I-I & Miwa Y, Indian J pure app/ Phys, 26 (1988) 197.

7 Miwa Y & Machida K, J Alii chem Soc, 110 ( 1988) 51 83.

8 Kundsen R, Sala 0 & I-Iase Y, J molec Struct, 32 1 (1994) 197: 321 ( 1994) 187.

9 Pauling L, The nature of chelllical bond (Cornell University, Ithaca, New York) 1960.

10 Gussoni M, Castiglioni C & Zerbi G, J phys Chern , 88 (1984) 600.

11 I-I argittai M, Samdal S & Seip R, J molec Struct, 71 (1984) 147.

12 Walter W & Kubersky I-I P, Ann Chelll, 694 ( 1966) 352.