Indian Journal of ChemistryVol. 21A, October 1982. pp. 949-952

Force Field Calculations for In-plane Vibrations ofOxamide.Use of CNDO/Force Method

A ANNAMALAI & SURJIT SINGH-Structural Chemistry Group. Department of Chemistry, Indian Institute "OfTechnology, Madras 600036

Received 25 May 1982; revised and accepted 21 August 1982

Theoretical force field for in-plane vibrations of oxamide has been evaluated using CNDO/force calculations. The initialforce field is set up by taking the interaction and bending force constants of the CNDO/force field. and transferring thestretching force constants from force fields of chemically related molecules. The experimental force field is then obtained byleast squares refinement method considering the vibrational frequencies of H2NCOCONHz and Dz NCOCONDz. The bandassignments are proposed on the basis of potential energy distributions.

The IR and Raman spectra of oxamide have beenreported in several papers I - 5. Desseyn et al?measured the vibrational spectra of D2NCOCOND2

and H2NCOCONH2. However, the assignments ofDesseyn et al.3 for H2NCOCONH2 differ considerablyfrom those of Durig et a12. For oxamide, a Urey-Bradley force field was evaluated by Wallace andWagner". In the present study, redundancy-freeinternal valence force field (RFIVFF) has beenevaluated for this molecule based on the CNDO/force 7

MO as well as the least. squares refinementcalculations. The band assignments have been made onthe basis of potential energy distributions (PED).

Calculations and ResultsThe CNDO/force MO calculations are known7-

9

to overestimate the stretching force constants by abouta factor of 2-3.5. However, these calculations predictthe stretch-bend and bend-bend interactions as well asbending force constants reasonably well. The stretch-stretch interaction constants are exaggerated by about50 '.I~.The signs of interaction constants are in generalcorrectly predicted. In order to frame the initial forcefield matrix for the least squares refinementcalculations, the diagonal force constants can easily betransferred from chemically related molecules, butsuch a transfer is difficult in the case of interaction forceconstants since thelatterdepend on the geometry of themolecule. Since, as mentioned above, the interactionand bending force constants of CNDO force field arereliable, these constants are considered for the initialforce field after scaling down the stretch-stretchinteraction constants by a factor of 0.65 (ref. 9) and thestretching force constants are transferred from relatedmolecules. The force field is then refined usingvibration frequencies.

The CNDO/force calculations were performed usinga modified version 7 of the computer programme

CNINDO, of Pople and Beveridge to. Based onCNDO/2 calculations, the trans-structure of oxamideis found to be energetically more stable than the cis-structure. This is in accordance with the experimentalfindings that indicate trans-conformation for themolecules in crystalline Oxamide.'{i'". The moleculargeometry was optimised using the steepest descentmethod described in previous studies 7 - 9. Theoptimised geometry was considered as the referencegeometry, and the forces were calculated by distortingthe molecule slightly in the positive and negativedirections of the redundancy-free internal coordinates.The theoretical force constants were then obtainedfrom the forces by numerical differentiation 7 - 9. Theinternal coordinates of oxamide are given in Fig. I. Thecalculated CNDO force field is given in Table I.

The experimental force-field was evaluatedemploying Wilson's GF matrix method 13. The initialforce field was constructed as suggested above. The Gmatrix elements were computed using the X-raystructural parameters reported by Ayerst and Dukell.The refinements were carried out usingSchachtschneider's FPERT program 14 after including

Fig. I-Internal coordinates of oxamide

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INDIAN 1. CHEM., VOL. 21A, OCTOBER 1982

Table I-Force Fields of Oxamide"

Force Theor. Exp. Exp. Force Theor.constant CNDO/force FFI FF2 constant CNDO/force

FI, I 15.287 4.786 4.588 F6,6 14.937FI,2 1.110 0.7(C) 0.7(C) F 6.10 0.047FI,4 0.785 O.5(C) O.5(C) F 6.12 -0025FI,IO 0.215 1.094 0.346 F 6,14 0.\38FI,12 0.355 0.087 0.196 F 8, 8 14.617FI,I6 0.054 0.004 -0.090 F 8,10 0.039F2, 2 29.382 10.784 10.687 F 8.12 0.050F2, 3 -0.060 F 8.16 0.291Fl,4 2.625 1.7(C) 1.7(C) FIO,IO 0.725Fl,5 0.054 FIO,II 0.\31F2,I0 -0.460 -0.737 "':'0.521 FIO,I2 0.102F2,I2 -0.073 -0.008 0.215 FIO,I3 -0.048Fl,I3 0.106 O.oJI -0.00 FlO,I5 -0.021F2,I4 -0.037 FIO,I6 0.106F2,I6 -0.038 Fl2.12 0.945F4,4 20.300 7.292 7.046 F12.13 0.057F4, 5 -0.071 FI2,I4 0.020F4, 6 0.696 0.4(C) O.4(C) FI2,I5 0.016F4.11 0.067 0.400 0.259 Fl2,16 0.Q78F4.12 -0.464 -0.339 -0.125 FI2,I7 0.013F4,I3 -0.028 FI4,I4 0.532F4,I4 -0.318 -0.068 -0.123 F14.15 0.010F4,I6 -0.021 F16.16 0.533F4.17 -0.022

Exp. Exp.FFI FF2

5.951 5.954

6.124 6.126

1.634 1.4390.067 -0.1100.002 0.090

-0.038 0.275

0.100 0.0021.402 1.3450.032 -0.068

0.15(C) 0.15(C)

0.457 0.462

0.542 0.607

* Units: stretch-stretch in Mdyn A -', stretch-bend in Mdyn Rad -, and bend-bend in Mdyn A Rad-1

(C) contrained force constant values.

damped Ieast squares subroutines. Based on C 2h

symmetry of oxamide, the 24 normal modes can begrouped as 9Ag + 8B" + 4A" + 3Bg vibrations. The forcefield calculations were performed by considering the 17in-plane vibrations (Ag and Bu) together. Vibrationalfrequencies of H2NCOCONH2 and D2NCOCOND2

in the solid phase taken from ref. 3 were employed inthese calculations. The absolute weighting factors 15

were considered for the frequency parameters duringinteractions. Two types of experimental force fields riz,FFI and FF2 have been evaluated in this work. In FFIthe signs of interaction constants were kept unalteredduring refinement, since the signs obtained from theCNDO/force calculations are, in general, reliable.However, such a constraint on signs was lifted whilecalculating the force field F F2 with a view to improvingthe frequency fit. All the computations have beenperformed on an IBM 3701155 computer. Of the 28significant force constants considered for theexperimental force field, only 23 were allowed to refineusing 33 vibrational frequencies. The mathematicalconstraints imposed on the force fields are based on theresults of CNDO/force calculations as well as the forcefields of related molecules. Some of the stretch-stretchinteraction constants were kept fixed at the valuesobtained after scaling down the CNDO estimates by afactor of 0.65 (ref. 9). The experimental force fields are

950

included in Table I. The calculated and observedfrequencies are summarised in Table 2 along withPED.

DiscussionOesseyn et a/.3 assigned the bends at 833 and

444 cm -I in the Raman spectrum of oxamide to Agskeletal deformation modes and the 638 and 472 cm - 1

bends of IR spectrum to B" skeletal deformationmodes. The present normal coordinate analysisindicates that the ()CCN vibration belonging to the B"species should have a frequency which is about200 cm - I lower than the corresponding vibration ofAg species. But the B" carbonyl rocking mode shouldhave a frequency about 200 ern - 1 higher than thecorresponding Ag type mode. Satisfactory fit isobtained by assigning the bands at 526 and 334 ern - 1

to Ag and B; bCCN modes respectively, and assigningthe 444 and 638 em - 1 bands to Ag and B" pCO modesrespectively. A similar assignment was proposed byWallace and Wagner" using Urey-Bradley potentialfunction. Based on reasons similar to those discussedfor oxamide, the 495 and 310 cm - 1 bands of oxamide-d4 are assigned respectively to Ag and B" bCCN modeswhereas the 424 and 569 em - I bands are assignedrespectively to Ag and B" pCO modes. These skeletaldeformation modes (2Ay + 2B,,) of oxamide-a, were

ANNAMALAI & SINGH: FORCE FIELD CALCULATIONS FOR OXAMIDE

Obs. Calc.FF2

~v PED for FF2

Table 2-Calculated and Observed Frequencies (in cm - I) and PEO for OxamideCalc.FFI

H2NCOCONH2 molecule

A.3380 3389 -9 3390 - J() F8,8(99)3190 3219 -29 3219 -29 F6,6(IOO)1703 1694 9 1697 6 F2,2(60) FI,H24) FlO,10(17) FI6,16(12) FI,2( -II)1586 1584 2 1596 -10 F14,14(81) F4,4(21)1485 1490 -5 1484 1 F4,4(41) F2,2(24) FI2,12(24) FI4,14(15) FI,I(l2) F2, 4{-12)1102 1104 -2 1097 5 FI6,16(52) F4,4(23) F2,2(15)807 799 8 807 o FI,I(4\) Fl6,16(31) F4,4(14) FI2,12(14)526 538 -12 526 o FIO,IO(90) F12,12(12) FIO,13( -13)444 445 -I 445 -I FI2,12(66) FI,I(30) FI,12(10)

B.3380 3389 -9 3389 -9 F8,8(99)3190 3218 -28 3219 -29 F6,6(IOO)1656 1654 2 1653 3 F2,2(89)1600 1601 -I 1595 5 FI4,14{9\) F4,4(13)1343 1320 14 1334 9 F4,4(58) FI6,16(16) FI2,12(15)1104 1099 5 1109 - 5 FI6, 16(55) F4,4(22) F2,2(11)638 655 -17 642 -4 FI2,12(72) FI6,16(22) FIO,IO(13) F12,16( -13) FIO,13( -12)334 314 20 335 -I FIO,10(76) F12,12(10) FIO,13(1 \)

D2NCOCOND2 molecule

A.2540 2527 13 2528 12 F8.8(98)2364 2323 41 2322 42 F6,6(IOO)1661 1682 -21 1669 - 8 F2.2(74) FI,I(23) FIO,10(20) FI,2( -12) F2,I0( -10)1495 1487 8 1498 -3 F4,4(66) FI2,12(26) F2,2(l7) FI,I(15) F2,4( -13)1205 1198 7 1192 13 FI4.14(76)

945 947 -2 948 -3 FI6,16(27) F4.4(19) F14,14(16) FIO,10(14) F2,2(l3)719 690 ~- FI,1(4\) FI6,16(52)

495 491 4 492 3 FIO,10(81) FI2.12(27) F4,4(10) FIO.I3( -19)424 412 12 424 o FI2,12(52) FI,I(22) FI6.16(12)

B.2540 2528 12 2526 14 F8,8(98)2364 2323 41 2322 42 F6,6(IOO)1640 1640 0 1640 o F2,2(95) F2.4( - II)1376 1382 -6 1375 I F4,4(72) FI4,14{2\) FI2,12(1O)

1110 1119 -9 1119 - 9 FI4,14(66) F12,12( 12)

922 942 -20 928 -6 FI6,16(44) F4,4(14) FI4.l4(12)569 559 10 566 3 FI2,12(66) FI6 16(40) F12,16( -17)

310 288 22 309 I F I0,1 0(79)---.--.~ _ ... _--------_._- --"--, ..- ------,--- -- .~------,.-- ..- ..- .- ,--------~--

assigned by Desseyn ei at.' to the 828 and 424 cm-I

bands observed in the Raman spectrum and to the 621and 460 cm -I bands observed in the IR spectrum.Broad bands in the NH2 stretching region are assignedto both Ag and B; vibrations. The force constantsobtained, for the skeletal vibrations of oxamide withthe new assignments are reasonable and arecomparable to the corresponding force constants ofacetamide+". The CO, CN and CC stretching, CCNbending and CO rocking force constant values ofoxamide are found to be close to those reported in thesolid phase using calculations similar to those of thepresent work (for acetamide. the CO, CN and CCstretching, CCN bending and CO rocking forceconstants were found to be 10.779, 7.054, 4.503, 1.251and 1.283 mdyn A-I. respectively). The force constantsfor the NHz group vibrations of oxamide are also in

good agreement with the corresponding forceconstants of formamide and acetamide 16. Thefrequency fits obtained with the force fields FFt andF F2 are satisfactory; however, the fit is slightly betterwith FF2.

AcknowledgementThanks are due to CSIR and DAE for the financial

assistance given to A.A. We are grateful to theauthorities of the Computer Centre, lIT Madras fortheir help.

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(1971) 129.3 Desseyn H 0, Van Der Veken B J & Herman H A, Spectrochim

Acta, 33A (1977) 633.

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INDIAN J. CHEM., VOL. 21A, OCTOBER 1982

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6 Wallace F & Wagner E, Spectrochim Acta, 34A (1978) 86.

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8 Pulay P & Torok F, Molec Phys, 25 (1973) 1153.

9 Annamalai A & Singh S, Proc Indian Acad Sci, 87A (1978) 337,JMolec Struct Theoret chem, 87 (1982) 169.

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10 Pople J A& Beveridge D L, Approximate molecular orbital theory(McGraw-HilI, New York), 1970.

11 Ayerst E M & Duke J R C, Acta Crystallogr, 7 (1954) 588.12 De With G & Harkema S, Acta Crystallogr, 338 (1977) 2367.13 Wilson E B, Decius J C & Cross P C. Molecular vibrations

(McGraw-Hill, New York), 1955.14 Schachtschneider J H, Technical reports No 57-65 (Shell

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