Vibrational characterization of the tertiary amide and thioamide group

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Vibrational characterization of the tertiary amide and thioamide group H.O. Desseyn *, W.A. Herrebout, K. Clou Department of Chemistry, University of Antwerp-RUCA, Groenenborgerlaan 171, B-2020 Antwerp, Belgium Received 16 May 2002; received in revised form 27 June 2002; accepted 27 June 2002

Abstract Force field calculations and vibrational spectra of (CH3)2NCOCON(CH3)2 (TMO), (CH3)2NCOCSN(CH3)2 (TMMTO) and (CH3)2NCSCSN(CH3)2 (TMDTO) are discussed. The amide and thioamide fundamentals and those of other simple tertiary amides are compared. A characteristic pattern in infrared and Raman is proposed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Ab initio calculations; Vibrational spectra; Tertiary amides and thioamides

1. Experimental (CH3)2NCOCON(CH3)2 (TMO) was prepared by slowly adding (CH3)2NH to a toluene solution of ClOCCOCl (Merck 807066). (CH3)2NCOCSN(CH3)2 (TMMTO) and (CH3)2NCSCSN(CH3)2 (TMDTO) were prepared by reaction of (CH3)2NCOCON(CH3)2 with P4S10 in toluene solution and were separated on a Al2O3 column with toluene. The compounds were further purified by recrystallization from methanol. The infrared spectra were recorded on a Bruker IFS 113v Fourier transform spectrometer, using a liquid nitrogen cooled MCT detector, with a * Corresponding author. Tel.: /32-3-218-0365; fax: /32-3218-0233 E-mail address: [email protected] (H.O. Desseyn).

resolution of 1 cm 1. For each spectrum 100 interferograms were recorded and averaged. The low-temperature measurements were performed using a laboratory designed nitrogen cooled cryostat. The latter consists of a copper sample holder attached to a small container that can be filled with liquid nitrogen, the sample holder and container are surrounded by a jacket equipped with KBr windows and placed under vacuum. The farinfrared spectra have been recorded using a DTGS detector with a resolution of 4 cm 1. For each spectrum 250 scans were recorded and averaged. The Fourier transform Raman spectra were recorded on a Bruker IFS 66 v interferometer equipped with a FT Raman FRA 106 module. The molecules were excited by the 1064 nm line of a Nd:YAG laser operating at 200 mW. For each

1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 2 5 4 - 8

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recorded with a spectral slit of 4 cm 1. For each spectrum three scans were recorded and averaged. The figures and tables given in this article all refer to the low-temperature spectra, as these spectra generally give sharper bands. This is clearly shown in the variable temperature infrared spectrum of TMDTO in the 1600
/1475 cm1 region (Fig. 1).

2. Computational details

Fig. 1. The 1600
/1450 cm1 region of the infrared spectrum of TMDTO at different temperatures.

spectrum 1000 scans were recorded and averaged. The low-temperature Raman spectra were recorded in a SPEX 1403-0.85 m double monochromator. The molecules were excited by a Spectra Physics model 2000 Ar-ion laser. A Miller
/Harney cell [4] was used to cool the sample. The spectra were

The density functional theory calculations were performed using GAUSSIAN-98 [1]. For all calculations Becke’s three-parameter exchange functional [2] was used in combination with the Lee
/Yang
/ Parr correlation functional [3], while the 6-31G* basis set was used throughout, as a compromise between accuracy and applicability to larger systems. To reduce the errors arising from the numerical integration, the ‘finegrid’ option, corresponding to roughly 7000 grid points per atom, was used for all calculations. Since ab initio calculations refer to an isolated molecule, it would be appropriate to compare the calculated frequencies with the gas-phase frequencies of the compounds under investigation but the high boiling points of these compounds, given by (dm /dT )max from the TGA curve (190 8C for TMO, 227 8C for TMMTO and 253 8C for TMDTO) indicate a very low vapor pressure at room temperature, and the impossibility of any gas-phase spectra of the compounds.

Table 1 ˚) Theoretical and experimental bond lengths (A TMO Experimental [34] C /C C /Namide C/Nthioamide C /O C/S u (8)

TMDTO Calculated

1.530 1.319

1.542 1.363

1.228

1.231

79.4

59

Experimental [35]

TMMTO Calculated

1.518

1.510

1.310

1.346

1.672 89

1.675 76

Experimental [31]

Calculated

1.510 1.319 1.334 1.225 1.670 87.4

1.527 1.364 1.345 1.230 1.673 81

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Fig. 2. Low temperature infrared and Raman spectra of TMO and the normal (A) and polarized (B) aqueous solution Raman spectra of TMO.

Frequencies calculated with ab initio force constants are generally 10
/20% higher than the experimental frequencies [5], primarily due to limitations in the basis set and the neglect of electron correlation. Therefore, we have scaled the

force constants to better reproduce the observed frequencies. The scale factor used, 0.95, agrees very well with the data given in the literature [6,7] for all fundamentals, as these tertiary amide modes are not influenced by hydrogen bonding.

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Table 2 Calculated fundamentals, in cm 1, and P.E.D. values for (CH3)2NCOCON(CH3)2

n1 n2 n3 n4 n5 n6 n7 n8 n9 n10 n11 n12 n13 n14 n15 n16 n17 n18 n19 n20 n21 n22 n23 n24 n25 n26 n27 n28 n29 n30

ncalc.

P.E.D.

3051 3051 3031 3031 2933 2933 2929 2929 2890 2888 2882 2882 1652 1646 1492 1486 1455 1454 1453 1453 1434 1432 1426 1422 1392 1379 1379 1362 1229 1220

n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 69n CO, 10n CN 77n CO, 13n CN d CH3, 14n CN d CH3, 12n CN d CH3, 10n CN d CH3 d CH3 d CH3 d CH3 d CH3 67d CH3, 10n CN d CH3 d CH3, 19n CN d CH3 d CH3 30n CN, 15nsNR2 46nasNR2, 15r NR2, 13d NCO 57nasNR2, 17r NR2

n31 n32 n33 n43 n35 n36 n37 n38 n39 n40 n41 n42 n43 n44 n45 n46 n47 n48 n49 n50 n51 n52 n53 n54 n55 n56 n57 n58 n59 n60

As the present force fields can be regarded to be reliable, they can be used to characterize the tertiary amide and thioamide fundamentals.

3. Introduction The vibrational spectra of primary and secondary amides are probably the most studied and best characterized functional groups [8
/17]. However, only a few simple tertiary amides such as HCON(CH3)2 [18
/22] and CH3CON(CH3)2 [23,24] and NCCON(CH3)2 [25] have been studied, by Urey
/Bradley force fields, and disagreements on, for instance, the assignment of the nCN mode

ncalc.

P.E.D.

1169 1130 1129 1082 1082 1062 1037 1036 947 855 758 718 619 609 424 402 374 356 338 274 245 222 211 136 130 101 98 77 76 48

r CH3, 16nasNR2, 14n CC r CH3 r CH3 r CH3 r CH3 r CH3, 31n CN r CH3, 19nsNR2 r CH3, 19nasNR2 49nsNR2, 10n CC 57nsNR2, 20d NCO 79p CO, 12r NCO 24r NCO, 17p CO, 13n CC, 13nsNR2 38d NCO, 18nsNR2, 12r NR2, 10n CN 30p CO, 23nsNR2, 19n CN 33d NR2, 29r NR2, 13p CO, 10r NCO 79d NR2 40d NCO, 15d NR2, 12r NCO, 11n CC 52r NR2, 27r NCO, 13d NCO 29r NR2, 28d NR2, 26n CC 29p CN, 19r NCO, 14d NCO 74p CN, 10r NCO 44r NCO, 19t NR2 39p CN, 17r NCO, 15t CC, 10tasCH3 tsCH3, 10t NR2 tsCH3 40t NR2, 30p CN, 10t CC tasCH3, 28p CN, 21t NR2 61t NR2, 14p CO tasCH3, 20r NCO, 20tsCH3, 12t NR2 58t CC, 14t NR2

[18,20,26,27] indicate that the characterization of the tertiary amide group is highly complex. The characterization of the primary and secondary thioamide groups have also been the subject of numerous papers and the results of the various authors are frequently in disagreement even for the simplest thioamides [28
/31] and some typical bands designated as thioamide I, II, III. . . in analogy with the amide bands have been discussed [32]. The nature of the tertiary thioamide bands is very complex [18,33] and only some simple molecules such as HCSN(CH3)2 [18] and CH3CSN(CH3)2 [33] have been studied. We already assigned some fundamentals in TMO [12]

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Table 3 Experimental infrared and Raman data, in cm1, and assignments for (CH3)2NCOCON(CH3)2 Infrared solid (/120 8C) 3272 w 3156 br w 3097 vw 3057 vw 3012 m 2197 w 2934 ms 2922 sh 2887 w 2862 mw 2795 w 1770 w 1653 vs 1637 vs 1569 vw 1552 m 1527 w 1496 ms 1473 w 1462 vw 1439 w 1432 m 1405 ms 1390 ms 1300 vw 1254 ms 1223 sh 1200 ms 1159 vw 1100 s 1065 m 974 m 878 m 850 sh 783 s 770 s 663 mw 649 m 442 w 415 w 377 sh 362 ms 259 m 228 mw 125 brw 84 mbr

Raman solid (/120 8C)

Raman H2Osoln.

3150 (1) 3050 sh 3012 (3) 2972 (3) 2940 (10) 2911 sh 2867 (2) 2804 (3) 1661 (2) 1641 (6) 1560 (1)

1652 (4) P 1625 (2)

1525 (3)

1533 (7) P 1510 (1) 1470 sh 1468 (7) P 1452 (2) P 1428 sh 1403 (5) P

1464 (8) 1443 (3) 1411 (3) 1394 (2)

(3) (1) (3) (2) (3) (2)

1365 (1) P 1300 (2) P 1268 (2) P 1235 (1) 1203 (3) P 1155 (2) 1111 (4) 1061 (2) 977 (4) 890 (3)

772 (6) 670 (4) 650 (4)

788 (6) P 770 (2) 685 (10) P 652 (4)

1300 (1) 1267 (2) 1199 1157 1106 1066 971 882

443 414 375 364

(3) (1) (1) (4)

263 (2) 219 (1) 203 (3) 124 (2) 89 (10) n.o. n.o. n.o.

446 (3) P 372 sh 368 (8) P 330 sh 250 (1) P 243 sh 220 sh 210 (3)

Assignments (cfr. Table 2) 2/n13 n13, n15 2/n15 n1, n2 n3 n4 n5, n6 n7, n8 n9, n10 n11, n12 2/n25 2/n40 n13 n14 2/n9 n15 n16 n17, n18, n19, n20 n21, n22 n23 n24, n25 n26 n27 n28 /comb. band n29 n30 n31, n32 n33, n34, n35 n36, n37, n38 n39 n40 comb., band n41/comb. band n42 n43 n44 n45 n46 n47 n48 n49 n50 n51 n52 n53 n54, n55 n56, n57 n58 n59 n60

C2 symmetry

nA,1 nA,2

A B

nA,3

A

nA,4

B A

nA,5 nA,6

B

nA,7 nA,8

B A

nA,9

A

nA,10 nA,11 nA,12 nA,13 nA,14 nA,15 nA,16 nA,17 nA,18 nA,19 nA,20 nA,21

A B A A A B A A A B B A

nA,22 nA,23 nA,24

A B

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Table 4 Comparison of the calculated amide fundamentals, in cm 1, and P.E.D. values of (CH3)2NCOCON(CH3)2 and (CD3)2NCOCON(CD3)2 (na,x: amide bands given in Table 3) ncalc.

P.E.D.

(CH3 )2 NCOCON (CH3 )2 nA,1 1652 69n CO, 10n CN nA,2 1646 77n CO, 13n CN 1492 64d CH3, 14n CN nA,3 nA,4 1362 30n CN, 15nsNR nA,5 1229 46nasNR2, 15r NR2, 13d NCO nA,6 1220 57nasNR2, 17r NR2 nA,7 947 49nsNR2, 10n CC nA,8 855 57nsNR2, 20d NCO nA,9 758 79p CO, 12r NCO 718 24r NCO, 17p CO, 13n CC, 13nsNR2 nA,10 nA,11 619 38d NCO, 18nsNR2, 16r NR2, 10n CN nA,12 609 30p CO, 23nsNR2, 19n CN nA,13 424 33d NR2, 29r NR2, 13p CO, 10r NCO 402 79d NR2 nA,14 nA,15 374 40d NCO, 15d NR2, 12r NCO, 11n CC nA,16 356 52r NR2, 27r NCO, 13d NCO nA,17 338 29r NR2, 28d NR2, 26n CC 274 29p CN, 19r NCO, 14d NCO nA,18 nA,19 245 74p CN, 10r NCO nA,20 222 44r NCO, 19t CN nA,21 211 39p CN, 17r NCO, 12t CC 101 40t NR2, 30p CN, 10t CC nA,22 nA,23 77 61t NR2, 14p CO nA,24 48 58t CC, 14t NR2 (CD3 )2 NCOCON (CD3 )2 nA,1 1629 72n CO, 10n CN nA,2 1626 79n CO, 12n CN nA,3 1399 50n CN, 14n CC, 11d NCO nA,4 1335 53n CN, 10n NR2, 9d NR2 nA,5 1201 57nasNR2, 13r NR2, 10d NCO nA,6 1195 62nasNR2, 14r NR2 nA,7 895 16n NR2, 14d NCO nA,8 809 27r CD3, 20nsNR2 nA,9 739 58p CO, 12nsNR2 nA,10 693 28r NCO, 23p CO, 13n CC, 10nsNR2 nA,11 581 38d NCO, 20nsNR2, 12n CN, 11r NR2 nA,12 571 27pCO, 25nsNR2, 22n CN nA,13 382 27r NR2, 22r NCO, 19p CO, 12d NR2 nA,14 341 50d NR2, 21r NR2, 13r NCO nA,15 355 35d NCO, 26n CC, 22d NR2 nA,16 330 37r NR2, 24r NCO, 10d NCO nA,17 297 41d NR2, 29r NR2, 14n CC nA,18 264 26r NCO, 20p CN, 12d NCO nA,19 217 57p CN, 21r NCO 197 37p CN, 20r NCO, 17t CN nA,20 nA,21 186 55p CN, 18t CC, 11r NCO nA,22 80 39t NR2, 23p CN, 16t CC nA,23 60 51t NR2, 16p CO 43 43t CC, 23t NR2, 10p CN nA,24

and TMDTO [28], but no information on the vibrational spectroscopy of TMMTO has been published. All compounds under investigation exhibit methyl groups. As the vibrations localized in these groups are sufficiently known, they are only mentioned in the tables and only the fundamentals involved in the tertiary amide and thioamide groups are further discussed.

4. Molecular geometry Dihedral angles between the two tertiary amide, or thioamide, groups have been measured from Xray data to be 71.58 for TMO and 87.48 for TMMTO [34], and molecular mechanics and U.V. studies indicated a stable conformation for TMDTO for a dihedral angle between 87 and 898 [35,36]. Table 1 gives a comparison between the optimized geometry data in our calculations and the experimental values of the most important bond lengths and the dihedral angle u . Also from the theoretical values, we can conclude that a greater value for u results when oxygen is replaced by sulfur, in agreement with the literature data [34,37]. As expected, the CN bonds are considerably shorter than the single C(sp2)
/N(sp2) bond ˚ ) [38]. Also the CS and CO bonds length (1.470 A are between the values for single and double bonds. These facts demonstrate the importance of the charged resonance form (B) in the tertiary structures. The central CC bond is considerably longer ˚ proposed by compared with the value of 1.466 A M.G. Brown [39] for a normal C(sp2)/C(sp2) bond, it is known [40] that when each of these sp2 carbon atoms is connected to a substitutent which contains delocalized electrons, the CC bond length is increased. The significant contribution of the polar resonance form, and the non planarity of these molecules is also reflected from the dipole moment and the great values of the rotational

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Fig. 3. Low temperature infrared and Raman spectra of TMDTO.

barriers around the carbon
/nitrogen bond, 96
/ 100 kJ mol1 [41]. We measured these dipole moments according the method of E.A. Guggenheim [42] in benzene solutions. The resulting values are: mTMO /3.87 D, mTMMTO /4.56 D and mTMDTO /5.30 D. This sequence agrees with the literature data which indicate a greater dipole moment for the thioamide structure [43]. Assuming a dihedral angle of 908, we calculated from TMO and TMDTO the partial moments of the tertiary groups to be mCONR2 / 2.74 D and mCSNR2 /3.75 D. Incorporating these data, and assuming u is 908 for TMMTO, we calculated a dipole moment mtheor. TMMTO /4.59 D, which is very close to the experimental value of 4.56 D. These comparable dihedral angles, and the evidence in the vibrational spectra (A and B

modes) let us consider a C2 symmetry for TMO and TMDTO and a C1 structure for TMMTO.

5. Vibrational analysis 5.1. (CH3)2NCOCON(CH3)2 (TMO) The low temperature solid state infrared and Raman spectra and the Raman solution spectra in H2O are for TMO given in Fig. 2. Table 2 schedules the 60 calculated fundamentals and their P.E.D. values. These fundamentals can be classified in 12 n CH3, 12 d CH3, 8 r CH3 and 24 other fundamentals (i.e. the fundamentals considering the methyl group as a point mass). The CH3 vibrations are very well known and are given in the tables without further discussion. For

842

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Table 5 Calculated fundamentals, in cm 1, and P.E.D. values for (CH3)2NCSCSN(CH3)2

n1 n2 n3 n4 n5 n6 n7 n8 n9 n10 n11 n12 n13 n14 n15 n16 n17 n18 n19 n20 n21 n22 n23 n24 n25 n26 n27 n28 n29 n30

ncalc.

P.E.D.

3033 3031 3004 3004 2974 2974 2961 2960 2914 2914 2902 2902 1491 1476 1453 1452 1440 1438 1435 1435 1426 1425 1397 1397 1378 1364 1252 1226 1154 1124

n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 35n CN, 35d CH3, 14r CH 46d CH3, 26n CN, 11r CH3 d CH3 d CH3 d CH3 d CH3 d CH3 d CH3 d CH3 d CH3 d CH3 d CH3 d CH3, 25n CN, 10nsNR2 d CH3, 32n CN, 13nsNR2 28nasNR2, 22r NR2, 13d NCS, 12n CC 45nasNR2, 21r NR2 49r CH3, 19n CC, 11nasNR2 57r CH3, 14n CN, 13n CS

the other fundamentals, only P.E.D. values of 10% and higher are given in the table. These 24 amide fundamentals can be represented, assuming the molecular C2 symmetry, in 13 A and 11 B modes. Table 3 gives the experimental solid state infrared and Raman spectra and the Raman H2O solution spectra. All calculated frequencies, as given in Table 2 are tentatively assigned (n1
/n60) and the 24 amide fundamentals have been indicated by nA1
/nA24, each exhibiting A or B symmetry. The assignments have been made in agreement with the literature data, with the calculated

n31 n32 n33 n34 n35 n36 n37 n38 n39 n40 n41 n42 n43 n44 n45 n46 n47 n48 n49 n50 n51 n52 n53 n54 n55 n56 n57 n58 n59 n60

ncalc.

P.E.D.

1112 1109 1098 1069 1068 1028 1026 956 888 800 617 590 556 503 424 414 351 282 281 262 212 194 163 128 125 103 91 77 76 52

r CH3 r CH3 29n CS, 27r CH3, 14n CC, 10d NCS r CH3 r CH3 r CH3, 29nsNR2 r CH3, 35nasNR2 44n CS, 13nsNR2 62nsNR2, 11n CC 63nsNR2, 15d NCS 67p CS, 21r NCS 45p CS, 21r NCS, 11nsNR2 33n CS, 14p CS, 14d NR2, 11nsNR2 26d NR2, 25d NCS, 22r NR2, 10n CS 39r NR2, 30d NR2 55d NR2, 20r NR2 36d NR2, 34r NR2, 18n CC 60p CN, 19r NCS 37p CN, 25r NCS, 20d NCS 34p CN, 30d NCS, 30r NR2 71d NCS 23r NCS, 21t CH3, 11p CS, 10d NR2 25r NCS, 21v NR2, 15p CS, 14t NR2 32tsCH3, 27tasCH3, 11d NR2 39tasCH3, 18tsCH3, 10v NR2 25t NR2, 22tsCH3, 13tasCH3 57tsCH3, 22p CN 43tasCH3, 20t NR2, 19p CN 36t NR2, 21tsCH3, 19tasCH3 68t CC, 14t NR2

frequencies as given in Table 2, and taking into account that all A modes can appear as polarized bands in the Raman solution spectra. From Table 3, we can see that the proposed assignments agree very well with the calculated frequencies, except for the nCN mode. The complex character of the nCN mode in tertiary amides has already been mentioned [18,20,26,27]. We calculated considerable n CN contributions in ten fundamentals in the 1650
/600 cm 1 region, especially coupled with methyl deformation modes and to a smaller extent (n43, n44) with amide deformation modes. In the Raman solution spectra, given in Fig. 2, an intense polarized band is observed at 1533

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Table 6 Experimental infrared and Raman spectra, in cm 1, and assignments for (CH3)2NCSCSN(CH3)2 Raman solid (/120 8C)

Infrared solid (/120 8C)

Assignments (cfr. Table 5)

3016 sh 3008 w 2953 w 2928 m 2890 vw 2867 w 2791 w 1644 vw 1614 vw 1562 sh 1537 vs 1521 vs 1463 vw 1447 sh 1440 m 1425 sh 1408 s 1403 s 1386 s 1281 s 1263 w 1249 s 1187 ms 1156 sh 1138 sh 1126 vs 1099 sh 1054 mw 988 ms 903 vw 826 ms 649 vw 628 ms 574 ms 519 w 433 s 374 w 310 s 223 w 179 m 176 sh 126 sh 109 m 87 w

3012 (2)

n1, n2 n3 n4, n5 n6, n7, n8 n9, n10 n11, n12 n13 /n27 2 /n40 n38 /n42

2959 2930 2890 2865 2801

(2) (8) (8) (1) (2)

1532 (2) 1526 sh 1456 (4) 1439 (4) 1405 (6) 1384 (2) 1282 (3)

1186 (2) 1154 (1) 1123 (4) 1099 (1) 987 909 828 650

(3) (3) (3) (4)

576 (10) 520 (3) 436 (4) 378 (3) 309 (1) 182 (4) 132 101 (10)

cm 1. This band cannot be due to n CO modes, which are clearly observed at 1652 and 1625 cm 1 in the polarized spectrum. Therefore, the 1533

n13 n14 n15 n16 n17, n19 n20, n23, n25, n27

C2 symmetry

nT1 nT2

A B

nT3

A

nT4

B

nT5

A

nT6 nT7 nT8 nT9 nT10 nT11 nT12 nT13, nT14 nT15 nT16, nT17 nT18, nT19 nT20 nT21 nT22 nT23 nT24

B A B A B A B A, B A A, B B, A B A A A B

n18 n21, n22 n24 n26

n28 n29 n30, n31 n32 n33 n34, n35 n36, n37 n38 n39 n40 n41 n42 n43 n44 n45, n46 n47 n48, n49 n50, n51 n52 n53 n54 n56 n60

cm 1 band is assigned as due to coupling and Fermi-resonance of the nCN mode and the CH3deformations.

844

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Fig. 4. Low temperature infrared and Raman spectra of TMMTO.

In order to observe the influence of the CH3 vibrations on the 24 amide fundamentals the vibrational frequencies of (CD3)2NCOCON(CD3)2 were also calculated from the ab initio force field. The results are given in Table 4. For nearly all fundamentals a downward shift due to the mass effect of the CD3 group, compared with the CH3 group is calculated. The P.E.D. values for the deuterated compound are similar to those of the normal compound, except for the n3 and n4 modes, where for the CD3 derivative two bands, at 1399 and 1335 cm 1, respectively, have been calculated with high nCN character. It has also to be noticed that for both isotopes, we calculated the same decreasing order in frequencies except for the n14 and n15, which have been interchanged in the CD3 derivative. There-

fore, from Table 3, we learn that, except for the nCN modes, the amide fundamentals are not strongly coupled with CH3 or CD3 vibrations.

5.2. (CH3)2NCSCSN(CH3)2 (TMDTO) The low temperature solid state infrared and Raman spectra of TMDTO are given in Fig. 3, while Table 5 schedules the 60 calculated fundamentals and their P.E.D. values. As the geometry of TMDTO is comparable with that of TMO, we can also consider 24 thioamide functions in a C2 symmetry (13 A and 11 B). These fundamentals are tentatively assigned (nT1
/nT24, each with A or B symmetry) in Table 6. The assignments have been made in accordance with the calculated values and the literature data [18,33].

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Table 7 Calculated fundamentals, in cm1, and P.E.D. values for (CH3)2NCOCSN(CH3)2 ncalc. P.E.D. n1 n2 n3 n4 n5 n6 n7 n8 n9 n10 n11 n12 n13 n14 n15 n16 n17 nn18 n19 n20 n21 n22 n23 n24 n25 n26 n27 n28 n29 n30

3042 3031 3024 3005 2975 2958 2938 2935 2912 2902 2892 2884 1649 1490 1481 1455 1454 1449 1440 1433 1431 1425 1421 1398 1384 1379 1366 1243 1224 1157

n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 n CH3 73n CO, 11n CN(O) 42d CH3, 26n CN(S), 17r CH3 d CH3, 12n CN(O) d CH3 d CH3 d CH3 dCH3 d CH3 d CH3 d CH3 d CH3 d CH3 d CH3, 14n CN(O) d CH3 36d CH3, 16n CN(O), 14n CN(S) 24nasNR2(S), 17r NR2(O), 12r CH3, 11nasNR2(O) 42nasNR2(O), 14r CH3, 13nasNR2(S), 11r NR2(O) r CH3, 18n CC

From the P.E.D. values in Table 5, we can also conclude that the thioamide n CN modes are coupled with the methyl deformations (n13, n14, n25, n26), but to a lesser extent than in TMO. The NR2 modes are, as expected, very well comparable with the data obtained for TMO, and also the thioamide deformation modes and n CC are strongly coupled. Five fundamentals in the region 1150
/500 cm 1 (n30, n33, n38, n43, n44 in Table 5) exhibit a considerable nCS character. Therefore, assigning a single fundamental characteristic for the nCS in tertiary thioamides is not possible.

ncalc. P.E.D. n31 n32 n33 n34 n35 n36 n37 n38 n39 n40 n41 n42 n43 n44 n45 n46 n47 n48 n49 n50 n51 n52 n53 n54 n55 n56 n57 n58 n59 n60

1129 1123 1109 1079 1068 1036 1032 1017 917 827 707 659 605 531 424 406 359 344 276 249 239 206 177 135 111 105 83 78 52 45

r CH3 r CH3 r CH3 r CH3 r CH3 r CH3, 18n?sNR2(O) r CH3, 18nasNR2(S) r CH3, 20n CS, 19nasNR2(S), 12nCN(O) 28nsNR2(O), 24nsNR2(S) 43nsNR2(S), 22nsNR2(O) 53pCO, 14pCS 23d NCO, 12p CO, 10n CS, 10nsNR2(O) 32p CS, 15r NCO, 13nsNR2(O), 10n CN(O) 22d NR2(S), 18n CS, 10r NCS, 11nsNR2(S) 33r NR2(S), 31d NR2(S) 48d NR2(O), 15r NR2(O), 10d NR2(S) 33r NCO, 23r NR2(O), 11p CS 21d NR2(O), 20r NR2(S), 19n CC, 17d NR2(S), 11r NR2(O) 35p CN(S), 14r NR2(O), 14r NCS 31p CN(O), 20r NCS, 20v NR2(S) 30r NCS, 18d NCS, 16v NR2(O) 20d NCS, 16t CH3(O), 13v NR2(O), 12t NR2(O) 17d NCO, 14v NR2(S), 12v NR2(O), 12p CS, 10t CC 74tasCH3(O) 47tsCH3(S), 14t NR2(S), 10d NR2(S) 47tsCH3(O), 14v NR2(O) 45t NR2(O), 14tasCH3(S) 31tsCH3(S), 24t NR2(S), 15v NR2(S) 28tasCH3(S), 14tsCH3(S), 12t CC 20t CC, 20tsCH3(S), 16tsCH3(O)

The thioamide deformation modes are strongly coupled with the NR2 deformations and none of these fundamentals is really characteristic for the tertiary thioamide group.

5.3. (CH3)2NCOCSN(CH3)2 (TMMTO) The low temperature infrared and Raman spectra of TMMTO are given in Fig. 4, Table 7 gives the 60 fundamentals and their P.E.D. values, while Table 8 schedules the spectra and the proposed assignments considering molecular C1

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Table 8 Solid state infrared and Raman spectra, in cm 1, and assignments for (CH3)2NCOCSN(CH3)2 Roman solid (/120 8C) 3270 3080 3009 mw 2955 mw 2926 m 2906 w 2870 w 2795 w 1692 vw 1650 vw 1638 sh 1544 vs 1504 m 1466 w 1411 w 1409 s 1393 s 1274 m 1257 s 1213 vw 1185 m 1145 s 1100 vw 1055 ms 1041 ms 938 w 860 sh 856 m 731 705 sh 702 s 646 mw 547 m 432 m 428 sh 375 s 361 s 2303 w 271 ms 250 sh 195 sh 187 w 118 sh 90 m 73 w 59 w

Infrared solid (/120 8C)

Assignments (cfr. Table 7)

3013 (2) 2954 sh 2932 (8)

2 /n13 n13 /n14 n1, n2, n3, n4 n5 , n6 n7 , n8 n9, n10 n11, n12

2875 sh 2807 (3)

C1 symmetry

1642 (5)

2 /n40 n13

1543 (3) 1499 sh 1461 sh 1446 (5) 1412 (7) 1399 sh 1275 (1) 1258 (2)

n14 n27 n15 n16, n17, n18, n19, n20 n21, n22, n23 n24, n25, n26 n28 n29

1185 (2) 1150 (3) 1106 (1) 1054 sh 1043 (3) 938 (2)

n30 n31, n32 n33, n34 n35, n36, n37 n38 n39

A

857 (2) 732 (1)

n40 n41

A A

702 (6) 649 (4) 549 (8) 438 (4) 430 sh 375 (5) 362 sh 303 (1) 270 (1) 249 (1) 198 sh 189 (2) 124 sh 101 (10)

n42 n43 n44 n45 n46 n47 n48

A A A A A A A

n49 n50, n51 n52 n53 n55 n56 n57, n58, n59 n60

A A, A A A A A

A A A

A A

A

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847

Table 9 Characteristic frequencies, in cm 1, for the tertiary amide group in X
/CONR2 molecules

X/H CH3 CONH2 CONHCH3 CON(CH3)2d CSNH2 CSN(CH3)2 CN COOK COOCH3 a b c d e

I.R. R. I.R. R. I.R. R. I.R. R. I.R. R. I.R. R. I.R. R. I.R. R. I.R. R. I.R. R.

n CO

n CNa

nasNR2

nsNR2

1684 vs 1680 (4) P 1640 vs 1640 (4) P 1640 s 1640 (3) P 1635 vs 1635 (3) P 1653 vs, 1637 vs 1661 (2), 1641 (6) 1626 vs 1625 (2) 1650 vs 1692 (5) 1680 vs 1678 (3) 1650 sh, 1600 vse 1650 sh, 1622 (6) 1661 vs 1660 (3) P

1512 m 1512 (2) P 1520 m 1508 (2) P 1505 mw 1520 (5) P 1532c 1538 (3) P 1527 v, 1496 ms 1533 (7) P, 1510 (1) 1538 m 1527 (3) 1504 m 1499 (2) 1530 mw 1535 (1) 1520 mw 1530 (2) 1502 m 1510 (2) P

1268 s
/ 1270 s 1269 (1) P 1254 ms 1265 (1) P 1236 ms 1236 (3) P 1254 ms, 1200 ms 1268 (2) P, 1203 (3) P 1233 m
/ 1257 s 1258 (2) 1262 s 1268 (1) 1260 ms 1265 (1) 1278 s 1280 (1)

870 m 882 (10) P 960 w, 737 vwb 962 (5) P, 740 (10) P 850 mw 846 (4) P 880 m, 870 sh 897 (10) P, 883 sh 974 m, 878 m 971 (3), 882 (2) 859 w 854 (4) 856 m 857 (2) 862 mw 850 (3) 840 m 841 (7) 862 m 866 (3) P

Fermi resonance n CN/d CH3. Coupling n CC/nsNR2. Overlapping with Amide II band. A and B bands in C2 symmetry. Overlapping with nasCO2 of carboxylate.

Table 10 Characteristic frequencies, in cm 1, for the tertiary thioamide group in X
/CSNR2 molecules

X/H CH3 CONH2 CON(CH3)2 CSNH2 CSNR2b CN COOC2H5 a b

I.R. R. I.R. R. I.R. R. I.R. R. I.R. R. I.R. R. I.R. R. I.R. R.

Coupling n CC/nsNR2. A and B in C2 symmetry.

n CN

nasNR2

nsNR2

1560 vs 1560 (2) P 1535 vs 1563 (4) P 1544 s 1541 (6) 1544 vs 1543 (3) 1546 vs 1550 (3) 1537 vs, 1521 vs 1532 (2), 1526 sh 1536 vs 1528 (5) 1548 vs 1549 (3)

1212 ms 1210 (1) P 1280 vs 1280 (1) 1260 s 1268 (1) 1274 m 1275 (1) 1286 ms 1292 (3) 1281 s, 1249 s 1282 (3) 1267 s 1271 (2) 1285 vs 1285 (2)

828 ms 830 (10) P 869 m, 820 vwa 870 (1), 834 (5) 900 m 910 (2) 856 m 857 (2) 851 m 853 (6) 903 vw, 826 m 909 (3), 828 (3) 905 mw 912 (3) 855 m 864 (5)

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symmetry (24 A modes regarding the methyl groups as a point mass). The P.E.D. values indicate strong coupling between the deformations of the NCO and NCS group and between the NR2 deformations of the amide and thioamide. From Table 7, it is clear that in this molecule, except the nCO, nCN, nasNR2 and nsNR2, we can only consider strongly coupled zone frequencies, which cannot be regarded as characteristic for the tertiary amide or thioamide function.

6. Conclusion Only a few bands are really characteristic for the tertiary amide group. The n CO appears as an intense band in infrared and of medium intensity in Raman in the 1650 cm 1 region. The n CN is strongly coupled with the methyl deformations and a characteristic, rather intense Raman band and less intense infrared band in the 1530 cm 1 region is also typical for the tertiary amide group. The NCO deformation modes are all strongly coupled and appear in the 800
/450 cm 1 region. Concerning the NR2 moiety of the amide and thioamide group it can be stated that the intense infrared band appearing in the 1250 cm/
1 region has mainly nasNR2 character, while the intense Raman band in the 800 cm 1 region exhibits high nsNR2 character. From the vibrational spectra of R2NCOCONR2 (where R /C2H5, C3H7, C4H9) we learn that these nsNR2 modes decrease by about 20 cm 1 when the CH3 is replaced by the C2H5 group, and decrease further with the mass of the alkyl group is very small. The diagnostic bands of the tertiary thioamide function are located as intense infrared and less intense Raman bands at 1550 cm 1 for the nCN mode. The nasNR2 and nsNR2 modes can also be considered as typical bands while, all other deformations of the NCS and the NR2 groups appear as strongly coupled vibrations. Consequently, these fundamentals have no diagnostic value. Tables 9 and 10 show the characteristic bands observed in simple tertiary amides and thioamides with general formulae X-CYN(CH3)2 (Y /O or S).

It has further to be noted that there is no regularity in frequency, character or intensity for other fundamentals in these tertiary amide/thioamide groups.

Acknowledgements The authors thank Greta Thijs for the technical assistance.

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