Indian Journal of ChemistryVol. 16A, October 1978, pp. 834-838

Frequency Assignment & Normal Coordinate Analysis of0-Thiovalerolactam & Its Seleno Analogue

K. DWARKANATH & D. N. SATHYANARAYANADepartment of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore 560012

andA. ANAGNOSTOPOULOS

Inorganic Chemistry Laboratories, Aristotelian University, Thessaloniki, Greece

Received 27 January 1978; accepted 28 March 1978

The vibrational assignments for a typical cyclic thioamide with a cis-CSNH group, namelyI)-thiovalerolactam (TVL) and its seleno analogue, I)-selenovalerolactam (SVL) have been carriedout. The normal coordinate analysis of TVL, (N-deuterated) TVL-d1 and SVL have been madefor all the fundamental vibrations using a valence force function employing transferred forceconstants from I)-valerolactam and N-methylthioacetamide. The main interest is In the bandassignments for the thioamide and selenoamide groups. The results are discussed in relationto assignments in related molecules.

CYCLIC amides and thioamides, usually referredto as lactams and thiolactams, respectively,are a class of interesting compounds. The

current interest in these compounds may be attri-buted to the study of structural and electronicproperties of the peptide and thiopeptide linkageswhich may be either in a cis- or trans-configurationl.

Investigations on lactams are comparatively morenumerous than their thio-analoguesv" while muchless is known about selenolactams++.

The six-membered lactams may be consideredas typical" strain-free" cyclic amides-. Recently,infrared spectra of Il-valerolactam and its N-deuterat-ed species have been assigned with the aid ofnormal coordinate analysis employing a generalvalence force field", It was considered interestingto extend this study to its thio- and seleno-analogues. The main aim of the present study isto obtain support for the assignment of the bandsdue to the cis-CSNH- group. The infrared bandassignments for Il-thiovalcrolactam (TVL, 2-thio-piperidone) proposed with the help of frequencyshifts in N-deuterated TVL and by normal co-ordinate analyses are discussed. The infrared spectraof c-thiocaprolact am (TCL) and its N -deuteratedspecies (TCL-dl) have also been studied.

Materials and MethodsTVL was prepared by a known procedure I starting

from Il-valerolactam (Aldrich) and P4S10 (Riedel).The product was purified by repeated recrystalliza-tions from hot xylene and dried in vacuo overphosphorus pentoxide; m.p. 94-96°. TCL was pre-pared by a similar procedure- and purified byrepeated reorystallizations from xylene; m.p. 103-105°.

SVL was synthesized- from Il-valerolactam andphosphorus pentaselenide (which was generated-in situ by refluxing purified red phosphorus and

834

grey selenium powder). The sclenolactam was sepa-rated from the lactam by column chromatographyand elution with a 1: 1 mixture of benzene andethyl acetate and recrystallized from chloroform.

Infrared spectra - Infrared spectra of the solidsamples were recorded on a Carl Zeiss UR 10spectrophotometer in nujol mull and KBr pellet.The solution spectra of the compounds were recordedin CCI4. Far infrared spectra were recorded on aFourier transform FIR 30 spectrometer in the fonnof polyethylene pellets.

Normal coordinate analysis - The X-ray structureof TVL has not been reported. A molecular modelsimilar to the one described for Il-valerolactamby Roy-Lafon et al.", which was based on theX-ray structure of «-chloro-Scvalerolactam", wasassumed for TVL and SVL. The bond lengthsused are: N-H 1·022; C-H 1·090; C-S 1·708;C-Se 1·860; C-N 1·365; Ca-N 1·495; C-C/X1·520; C -Cfl 1·540; Cfl-Cy 1·543 and Cy-Cs1·524 A.. The C-S bond length was transferredfrom imidazolidine-z-thionee and the C-Se bondlength was taken from selenourea". The bond anglesare: HCC 109·47; HCH 109; NCC 114·8; NCS123·2; CNH 115·4; CC/XC113·8; CCpC 108·5; CCyC110·3; CCsN 111-5 (in degrees).

The structure and internal coordinates of TVLand SVL are depicted in Fig. 1. TVL has 42normal vibrations and all are infrared - as well asRaman-active. The internal symmetry coordinateswere constructed by the standard procedure and arenot given here for brevity. The valence forceconstants of o-valerolactam were transferred to TVLand for the -CSNH- group the valence forceconstants were derived from the Urey-Bradleyforce constants of Nsmethylthioacetamidew-P. Onlyvery minor modifications of the initial force constantswere made. In accordance with the concept+'"of selenation, the force constants of TVL were

DWARKANATH et ai.: NORMAL COORDINATE ANALYSIS OF 8-THIOVALEROLACTAM

c

sFig. 1 - Structure and internal coordinates of 8-thiovalero-lactam [Torsional ('t")and out-of-plane bending (tt) coordinates

arc formed from bond numbers]

transferred to SVL with modifications only in the caseof C=S and N -H stretching constants (Table 1).The overall agreement between the observed andcalculated frequencies for TVL-do and -dt as well asthose of SVL, in spite of the zeroth order natureof the coordinate treatment, is quite satisfactory.

Results and DiscussionThe observed and calculated infrared frequencies

of TVL and TVL-dl are given in Table 2 andthose of SVL in Table 3 together with their mainassignments.

The assignments for the CH2 and C-C groupvibrations (Table 2) agree with those made for8-valeI'olactam5,13. Because of the main interestin the characteristic bands of the -CSNH- group,only these assignments will be discussed here.

- CXNH - group vibrations

The vibrations of the -CXNH - (X = S, Se)may be represented by three stretching (vNH, vCN,vCX), two in-plane bending (8NH, 8CX), and twoout-of-plane bending modes (7tNH, 7tCX).

N-H stretching - The infrared spectrum of TVLin the solid state shows two bands of almost equalintensity at 3180 and 3095*. The 3180 band isassigned to the N -H stretching of the associatedTVL molecules according to Hallam and jones-.The 3095 band of TVL is attributed to an overtoneof the NH bending occurring at 1582, intensifieddue to Fermi resonance with the 3180 fundamental'.The bands at 3180 and 3095 vanish in the spectrumof TVL-dt and are replaced by a strong and broad

*IR band positions are given in em? throughout thepaper.

TABLE 1 - FORCE CONSTANTSOF 8-THIOV'ALEROLACTAMAND 8-SELENOV'ALEROLACTAM

No. Force Value No. Force Valueconstant+ constant=

Stretch Stretch-bend

vNH 5-600 27 vCS, 8NH 0'080(5'70) 28 -cc-. 8NH 0·110

2 vaCH. 40650 29 -cc-, 8CS 0'0803 v"CH. 4-800 30 vCN, scs 0'1904 vCX 4·550 31 vCC.8HCH -0·284

(4-050) 32 vCC, wHCCoc 0·1905 vCC 3'800 33 vCC,8CCC 0'0356 vCN 5'850 34 vCN, 8CCN 0'0357 vCaN 4'450 35 -cc-. 8CCN 0'035

Bend Bend-bend

8 8NH 0'550 36 8"'H, 8CS 0'0009 8CX HOO 37 8CS, 8CCocCJ3 0'076

10 8HCH 0'540 38 8CS, wHCocC 0'02111 8HCocH 0'530 39 8(HCH)oc 0'009412 8HCaH 0'540 40 8HCocH, } 0'03013 8CCC 1'100 wHCocC14 8CNC 0'850 41 8CCC, 8CCC 0'38015 8CCN 1·200 42 8CNC,8CCC 0'38016 tCH. 0·650 43 8CCN,8CCC 0-38017 rCH. 0·740 44 8CNC, 8CocCN 0'39018 wHCC 0·580 45 -cs, rHCocH 0'13019 ttNH 0·330 46 ttNH, ttCS 0'11020 ttCX 0'125 47 ttNH, rHC8H 0·140

Torsion Bend-torsion

21 't"C8"'" 0·007 48 ttNH, 't"CyCa 0'03622 't"CN 0'018 49 rCH., -cc -0'00323 -cc, 0'028 50 ttCS, -recoc -0'02424 -rec 0'007

Stretch-stretch

25 vCC, -cx 0·60026 'leN, -cc, 0·490

·X = S or Se.Force constant Nos 1-7, 25 and 26 are in mdynl A; force

constant Nos. 27-35 are in mdyn : and the rest are in mdynA. The force constant of SVL when different from that ofTVL is given in the parenthesis.

w = wagging,v = stretching, 8 = bending, r = rocking,tt = out of plane bending and 't"= torsion (cf. Tables 2and 3).

band at 2345 due to N -D stretching, in the solidstate. Similarly TCL exhibits an intense band at3190 in the solid state which on deuteration isreplaced by a strong band at 2362. The N-Hstretching band of SVL is observed at a higherfrequency (3280) in the solid phase indicating thepresence of a much weaker hydrogen bonded N-Hgroup. Unlike TVL, the spectrum of SVL exhibitsonly one broad band above 3000. In CCl1 solutionTVL exhibits a sharp band at 3385 assignable tothe unassociated N -H stretching mode, indicatingthe presence of strong hydrogen bonded N-Hgroup in the solid state.

C- N stretching and N - H bending - The thio-amide-If band (N- H bending) and the thioamide-IIIband (C-N stretching) occur as mixed vibrations.The C- N stretching contributes principally to thebands at 1582 and 1365, and to a lesser extent

INDIAN J. CHEM., VOL. 16A, OCTOBER 1978

TABLE 2 - OBSERVEDAND CALCULATEDFUNDAMENTALS(em-I) AND ASSIGNMENTSFOR B-THIOVALEROLACTAM-dOAND -d1TVL-dl TVL-do PED*, %

Obs. Calc. Obs. Calc.

2345 2339 3180 3186 vNH(D) (100)2965 2973 2962 2973 vaCHa

2967 29672961 29612957 2957

2945 2930 2946 2930 vsCHa2873 2924 2875 2924

2922 29222917 2917

1582 1593 aNH (42), vCN (23), vC(jN (12)1547 1535 vCN (25), vCaN (13), wCetH, (13)1472 1497 1480 1504 aCIlH. (35), aC(jH, (18), wC,sH, (12)1458 1462 1462 1474 scsn, (20), wCetHa (15), VCetCfl(14), wCetH. (12)1458 1440 1432 1448 scsn, (24), aCyH, (21), wCaH. (14)1432 1434 1432 1446 sc-n, (29), aC(\H, (25), aCflH. (13)1412 1411 1410 1412 aCetH, (74)1365 1367 1365 1379 sc-rr, (39), wCIlH, (24)1330 1326 1365 1343 vCN (25), wCyH, (13), aNH (12)1280 1279 1330 1315 wCflH. (26), aCflH. (21)

1285 1278 WCflHa (30), wCetH, (18), aCetH, (13)1258 1231 1255 1224 tCetH, (51), tCyH, (21)1212 1219 1255 1214 tC8H, (68), tCflH, (12), tCetH, (to)1212 1204 tCaH, (34), vCCet (12)1170 1191 1180 1192 tCyH. (54), tCetH, (16)1122 1165 1173 1177 tCflH, (59), tCyH, (12), ecsn, (11)

1115 1113 vCCet (27), vCS (23), aCetCN (10)1072 1068 1100 1054 rCyH. (22), rCflH. (19), rCIlH, (17)1018 1034 vCS (30), aND (11)

952 967 vC(\N (30), wCaH, (23)950 945 930 942 rCaH, (30), rCIlH. (21), n-NH (14), rCetH, (13)930 915 930 90S vCflCy (23), WCflH' (16), wCyH. (11)895 891 rCyH, (14), VCflCy (12), wCflH, (11), 8ND (10)855 836 868 843 rCflH, (19), -c-rr, (18), n-NH (11), rCetH. (10)

868 830 vCyC" (34), VCflCy (13)855 808 830 810 rCetH. (41), VCetCfl(11), rCyH, (10)835 796 vCyCa (15), aND (13), vC~N (12), VCetCfl(11)

830 773 n-NH (45), rC8H, (12), aC(jH, (10)742 755 vCIlCy (22), VCetCfl(17)637 627 n-ND (65), aC8H, (13)

745 762 vCIlCy (23), VCetCfl(16)710

578 660 580 662 vCCet (16), vCN (13), 8CetCN (13), acc,c (10)455 433 456 436 aCCIJC (24), scc,c (20), aCNC (14), aCCaN (13)402 412 400 416 aCCaN (26), 8CCyC (21), aCNC (17)332 348 334 349 aCetCN (40), aCCyC (20)310 319 312 322 scs (34), aCCyC (15), accilc (11), aCC8N (10)262 273 283 275 aCCflc (30), aCNC (13)188 194 192 196 n-CS (52), n-NH(D) (20)108 128 107 136 "CN (28), "CaN (14), sccsc (13), n-NH (D) (11)84 90 85 91 n-CS (36), "CN (17), n-NH(D) (11)

·Potential energy distribution (FED) less than. 10 percent is excluded. The frequencies of TVL-dl are matched withthose of TVL-do when the FED values are approximately comparable. If the FED for TVL-d1 is significantly differentfrom that of TVL, then it is shown separately. Different symbols have the same meaning as indicated in Table 1.

to the band at 580. Corresponding to the bandsat 1582 and 1365 in TVL spectrum, bands may beidentified at 1560 and 1380 in TCL spectrum.This assignment of C-N stretching is comparablewith that in the case of thiazolidine-2-thione (Tzdt)l4,where it contributes to the bands at 1530 (52%)and 1356 (26%). In a-valerolactam, the nat~re ofC-N stretching is, however, different and It hascontributions ranging from 10 to 17% in a number ofbands", The present assignment of ~hioami~e~IIIband of TVL differs from the earher empiricalassignment by Mecke and Meckes at 1317 crrr '.The bands at 1590 (1575) and 1332 in the spectrum

836

of SVL have similar assignments as the 1582 and1354 bands of TVL.

The 1582 band (1592 in CC14) of TVL may bechiefly associated with a coupled N -H bendingmode. The N -H bending also contributes to theband at 1365 (12%). The 1582 band correspondsto the' B-band' of Jensen and Nielsen= both inposition and assignment, and resembles the strongband observed in the region 1550-1500 in othercyclic compounds like 2-thiopyrrolidone and Tzdt14,15containing cis -CSNH- group. A similar strongand broad band is observed at 1560 in the spectrumof TCL.

DWARKANATH et al.: NORMAL COORDINATE ANALYSIS OF 3-THIOVALEROLACTAM

The assignments of C-N stretching and N-Hbanding modes of TVL may be contrasted withthose in the case of N-methylthioacetamide (NMTA)which has a trans -CSNH- group, Suzukiw hasassigned, for:NMTA, a band at 1547 to a coupledvibration of NH bending and C-N stretching modesand another band at 1360 to a mixed mode ofsymmetric (C)CHa deformation, C-N stretchingand N-H bending by comparison with N-methyl-thioformamide, for:which normal coordinate analysishas been carried out. Thus, the assignments inTVL for C-N stretching and N-H bending modesare similar to those of NMTA showing vesy littledifference for cis and trans -CSNH- groups.But according to Rao and Chaturvedi1l, who havecarried out a normal coordinate treatment for:thein-plane vibrations of NMTA using a point massapproximation for the methyl groups, the N-Hbending and C-N stretching modes occur:as nearlypure vibrations and have been assigned respectivelyto bands at 1550 and 1440.

C=S and C=Se 1Jibrations - The band at --1115in TVL, Y-thiobutyrolactam and £-thiocaprolactamhas been empirically assigned to C=S stretchingby the earlier investigatorss-", Further, the C=Sestretching mode of SVLhas been assigned to a bandat 1085. It is satisfying to note that the potentialenergy distributions also show an appreciable contri-bution from C=S and C=Se stretching modesto the bands of TVL and SVL respectively at 1115and 1085. The C=S and C=Se stretching modeshave negligiblecontribution (lessthan 10°1,) to otherfrequencies. The assignment of C=S stretchingmode of TVL may be compared with that of Tzdtwhere it contributes 38% to the 1060 band, to amuch lesser extent to the bands at 1005 (13%)and 435 (18%).

The present study however shows that the exis-tence of a fairly pure C=S stretching mode forTVL is less probable. The downward frequencyshift of about 25 to 30 for TVL on "selenation"is also small to account for the occurrence of afairly pure C=S stretching mode. However, it isevident that the C=S stretching-mode contributesconsiderably to a band near 1110.

Hallam and Jones3 report that the 1110 bandof TCL shows a decrease of only 8 crrr+ onselenation compared to the corresponding shifts inY-thiobutyrolactam and TVL of about 27. Fromthis, it may be inferred that the C= S stretchingmode is more extensively coupled in TCL thanin the other two thiolactams.

According to normal coordinate treatment, theC=S in-plane bending of TVL may be assignedat 312 and the corresponding vibration of SVL at293. This assignment is compatible with the C=Sbending of Tzdt14,l5 at 292 and the C=Se bendingof thiazolidine-z-selenonewat 233. The C=S(Se)out-of-plane bending of TVL and SVL may heattributed to the bands at 192 and 228, respectively.The corresponding mode in Tzdt and its selenoanalogue has been assigned14,l6near 190.

N -H out-oj-plane bending - A band at 830 in theinfrared spectrum of TVL may be assigned to theN-H out-of-plane bending. For ~-valerolactam5.no definite assignment has been made for N-H

TABLE 3 - OBSERVEDAND CALCULATEDFUNDAMENTALS(em-I) AND ASSIGNMENTSFOR a-SELENOVALEROLACTAM

Obs, Calc. PED, %

3280 3214 "NH (100)2960 2973 vaCHa2940 2967

29612957

2865 2930 ",CHI292429222917

1590 1589 3NH (43), "CN (23). vCaN (12)15751528 1504 3C~H2 (35), aCaH. (18), wCyH. (12)15081478 1474 aCaH. (20), wCaH. (15). "CaC~ (14),

wC(lH. (12)1460 1448 scan, (24), 3CyH. (21), wCIlH. (13)1440 1446 IlCylls (29), scsn, (25), BC~1l2 (13)1410 1412 BCall. (74)1360 1379 BCyll. (39), wCBll. (24)1332 1340 "CN (24), wCyH. (15), IlNH (10)1318 1314 wCyll. (24), BC(llls (20), "CN (11)1272 1278 wC~Il. (30), wCaH. (19), IlCall. (12)1252 1224 tCaH. (51), tCyH. (21)1208 1214 tCaH. (68), tC(lH. (12), tCaH. (10)1185 1191 tCyH. (55), tCaHs (17)1172 1176 tCi*ls (61), ecsa, (12), tCyll. (11)1085 1098 "CCa (26), "CSe (18)1052 1051 rCyll. (21), rC~H. (16), rCaH. (16)1020 962 "CaN (27), wCaH2 (22)995920 942 rCall. (30), rC~H. (21), rCaH. (14), ",NH

(14)899 902 "C(lCy (23), wC(l1l2 (15), rCyll. (10),

wCyll2 (10)848 843 rC~ll. (19), rCylls (17), ",NH (11), rCaH.

(10)818 829 "CyCa (33), "C(lCy (14)790 810 rCaRs (41), "CaC(l (10), rCyH. (10)768 773 ",NIl (48), rCaH. (14), scan, (1l)750 758 "C~y (23), "CaC~ (20)597 658 "CCa (15), aCaCN (14), "CN (12), aCCaC

(10)432 436 aCC(lC (23), aCCaC (19), aCNC (15), aCCaN

(14)418 414 aCCaN (26), aCCyC (23). aCNC (16), scc,c

(10)364 347 aCaCN (38), scc,c (16), sese (11), aCNC355 (10), "CSe (10)293 321 sese (33), aCCyC (16), acc(lc (10)278 275 acc(lc (30), aCNC (13)228 196 -cse (52), ",NH (20)120 136 -rCN (28), -rC8N (14), sccsc (13), ",NH (11)90 91 -cse (36), TCCa (18), TCN (17), ",NH (11)

Different symbols have the same meaning as indicated inTable 1.

The 1582 band of TVL is replaced on N-deutera-tion by a new band at a slightly lower frequency(1547). Similarly, the 1560 band of TCL is replacedby a new band at 1525 in the spectrum of TCL-d1.The ND bending of TVL-dl is however, extensivelycoupled with other vibrations.

The single bonded C- N stretching mode is ob-served approximately in the same region for TVLand SVL. In the spectrum of TVL a band at 952is assigned to this mode but in SVL it is found

, at a higher frequency and bands at 1020 and 995contribute to this mode.

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INDIAN J. CHEM .• VOL. 16A. OCTOBER 1978

out-of-plane bending but it has been attributedto a band in the region 750-850 (calc. 796) and itcontributes to the bands at 796 (39%). 771 (16%)and 658 (28%). Thus the N-H out-of-planebending frequency of TVL is compatible with thatof ~-valerolactam. But it occurs at a higher wavenumber than that observed in other acyclic secon-dary thioamides having trans -CSNH - group(700-720)10.11.The 830 band of TVL decreases inintensity in the spectrum of TVL-d1 and a newstrong band appears at 637 as calculated. Similarly.the N-'-H out-of-plane bending of TCL may beassigned to a band at 845. which on deuterationalmost vanishes and a new band is observed at 630.

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