Rao, Ramana G

Reddy, Venkatram B

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InJian X Phys. 76B (4), 473-477 (2002)U P  B— an international journalNormal coordinate treatment of some pyridinesB Venkatram Reddy and G R ^ a n a  Rao*Department of Physics, University College, Kakatiya University, Vidyara|ya Puri, Warangal-S06 009, Andhra Pradesh, India•tbslract ■ A  53-paramcter modifledwalence force field has been obtained foe pyridine, pyridine-dS and 2,6-difluoro pyridine. The force i,onslam!. have been refined using vibrational frequencies employing Overlay-least-squares technique. Bigen vectors and potential energy distributinn.s were computed They have been used to propose unambiguous vibrational assignments for both in-plane and out-of-plane vibrational trcquencies of the three molecules. On this basis certain assignments proposed by earlier workers have been revised.keywords ' Normal coordinate treatmart, force constants, potential energy distribution.Ptrs No. 3,3 2 0 . - tI. Introduction1 ong tv a/ f I ] made zero-order nonnal coordinate calculations lor pyridine (PYR) and pyridine-d5 (PYR-d5) by transfering valence force constants from benzene. These computations suffer from the following short-comings.(i) Nitrogen atom in the benzene ring was treated as 14c as far as the force constants are concerned.(ii) The angle between the ;-th C-H bond and the exterior bisector of the C-C-C angle (or C-C-N angle) centred on the r-th carbon atom was defined as the internal coordinate B,, which account for C-H in-plane bending vibrations. This has been discarded in the contemporary literature by defining B’s interms of CCH angles themselves. Hence there is a need to incorporate this aspect into the computations.('•') The forece constants were not refined. Hence they can at best be an approximation.Therefore, we thought it worthwhile to re-do normal coordinate analysis for PYR and PYR-d5 eliminating the h^ove drawbacks. In order to increase the number of data o^od in the calculations we have included 2,6-difluoro pyridine for which an approximate nonnal coordinate analysis ^available in literature [2],' Address : University Post-Graduate College (Kakatiya University), Disk Karimnagar, Andhra Pradesh, India2. Normal coordinate treatment and resultsThe molecules under investigation possess C21. point group symmetry. In the C2v structure the 27 fundamentals of each of the three molecules are distributed as 10/41 + 9Bi + 3 A2 + 5Bi. The modes belonging toAu Bi and species are active both in infrared and Raman spectra, whereas the modes are active in Raman and forbidden in infrared. The structure parameters used are y(C-C) = 1.397 A, y(C-N) = 1.397 A, y(C-H) = 1.09 A, y(C-F) = 1.327 A and Z.CCC = ZCCN = ZCNC = ZCCH = ZCCF = 120°. Internal coordinates and symmetry coordinates used in the computations were adopted from monohalogenated nitrobenzenes [3] with suitable modifications for the present case. The initial force constants were trasferred from the final values reported in reference 3 and substituted anisoles [4], The initial constants connected with CNC moiety were taken to be equal to that o f CCC moiety. This initial set of 53 constants was subsequently refined through several cycles with a damping factor of 0.1 using overlay damped least-squares method [5], The vibrational frequencies required for the purpose were taken from Green et al [6], and Bailey and Steele [2J for PYR and DFP, respectively and u^cd those employed by Long [1] for PYR-d5. This allowed the approximate set of initial constants to adjust properly to the present set of molecules at the sameG odavarikhan i-SO S  2 0 9 .O 2002 lACS474Table 1. (Coni'd.).B Venkatram Reddy and G Ramana Raotime taking care of any correlations that may be present between certain constants. At this stage the damping factor was reduced to zero, all the interaction constants were fixed and refinement of 1S diagonal constants was continued using 78 vibrational frequencies of PYR, PYR-d5 and DFP. Under these conditions the refinement converged smoothly in three cycles with an average error of 11.1 cm ' between the observed and calculated frequencies. The final set of force constants thus obtain is given in Table 1 alongwith initial values and dispersions. The observed and calculated frequencies, potential energy distributions and vibrational assignment of the three molecules are presented in Table 2.Tabic 1. Force constants of pyridine, dcuterated pyridine and difluoro pyridine (in mdyn/A, mdyn/rad and mdyn-A/rad )^No Symbol CoordinatesinvolvedCommonAtomsValueInitiaF Final Dispersion(A) In-plane force constants Diagonal force constantsStretch1 Kj CC -  6.59572 Kp C-N -  6.59573 K, C-H ~ 5.18604 K., C-F -- 6 25676.3646 0.0426.6000 0.081 5.0997 0.0086.5060 0 127Bend5 ^CCC 0.8866 1.0874 0.0556 Ha dUCNC - 0.8866 1.0572 0 1067 lip /NCC - 0.8866 1.2741 0.0808 Hp ^CCH - 0.5111 0 5546 0.0069 Hp, ^NCH 0.5111 0.5269 001910 H* ZCCF 0.8775 1.2593 0.11411 Ha. ZNCF 0 8775 0.1382 0 080Interaction constants Stretch-stretch12 C-C,C~C C 0.9117 1.3080 0.013 C-C ,O C - -0.5506 -0.5521 OO14 P’W.P) C-C, C-C - 0.1882 0 0843 0.015 C-C, C-N c 0.9117 0 5805 OO16 C~C. C-H c -0,0264 -0,0124 0.017 C-N, C-H c -0.0264 0.2870 OO18 C-H. C-H - -0.0334 -00524 OO19 C-^ H, C~H -0.0664 -0.0261 OO20 H’fr.r) C-H, C-H - -0.0970 00273 0.0No Symbol Coordinates involvedCommonAtomsValue21 C-C, C-F C 0.238322 C-N, C-F C 0.238323 C-N, C-N N 0.9117Stretch-bend24 C-C, ZCCC C-C 0.417825 C-C, ZNCC C-C 0.417826 C-N, ZCNC C-N 0.417827 C-N, ZNCC C-N 0.417828 C-C, ZCCH C-C 0.157329 J'Wl) C-N, ZNCH C-N 0.157330 C-H, ZCCH C-H -0.022431 hr.4l) C-H, ZNCH C~H -0.022432 C-C\ ZCCF C-C -0.000633 C-N, ZNCF C-N -0.000634 C-F, ZCCF C-F 1.005935 C-F, ZNCF C-F 1 0059Bend-bend36 ZCCC, ZCCC C-C -0.040737 ZC:NC, ZNCC C-N -0.040738 ZCCC, ZNCC C-C -0.040739 ZCCH, ZCCH C-C 0 044040 ZCCH, ZFCC C-C -0.0308(B) Out-of-planc force constants41 s C C - 0.362242 Si C-N - 0.362243 n C-H - 0.458944 Yfi\ C-F - 0.657645 Yrm C-H, C-H C-C -0.069646 YRJt\ C-H, C-F C-C -0.073447 C-C.C-C ccc -0.011348 S,Yr C-C, C-H C-C 0.180549 C-C. C-N ccc -0.011350 <51 ,<51 C-N, C-N C-N -0.011351 Si,YR C-N, C-H C-C 0.180552 &Yr \ C-C, C-F C-C 0.221853 Si.rRi C-N, C-F C-C 0.2218initiar Final D,spe„,on0 8179 0 00.4165 0 00.2438 0 00.3347 0 00.0240 0 0-0.1290 0 00.3230 0 00.1817 0 00.1854 0 00.0449 0 00.1297 0 0-0.4730 0 00.3019 0 01 1590 0 00.5246 0 00 0860 0 0-0.1789 0 0-0.0651 0 00.0449 0 0-0,3214 0 00.2781 0 00420,3059 OOOlO0.4628 0 00220.6378 0 0103-0.0518 0.0-0.0541 00-0.0162 0.00.1573 00-0.0086 0.00.0196 000.1473 000.3243 0.00.0536 0.0 ^*Force constants at Nos. 18.19 and 20 are taken from ref [4].Normal coordinate treatment o f some pyridines 475fable 2. Observed and calculated frequencies (in cm ‘) and vibrational assignnient.Frequency (cm ’) PYR PYR-d5 2,6-DFPNO Modev(CH/CD) 2 v(CH/CD) 13/7fl v(CH/CD/CF) 20a v((TN) I v(CC) 8^ 7 v(CC) ]0a fKZCCC) 6a  Pi^ CCC) 12 ACH/CD/CF) 9a  fliCH/CD) 18flv(CH/CD/CF) 7b v(CH/CD) 20b v(C’N) U  \ { C C )  14 v(CC) \9b M/CCC) 6b AC'H/CD) 3 ACll/CD) 15/96 /iHCH/CD/CF) 186;t(CH/(:D/CF) lOfl 4C W C D ) \ 7a i(ZCCCC) \6 a«(CH/CD) 5 ;i(CH/CD/CF) 106 i^(CH/CD) 11 itZCCCC)4 KZCCCC) 166Obs.3036305430549921583148260510681218103030803036157213751439652128810851148891986375942886749700405Calc.3035326230809971595148660510601213103730863039156813701442649129910921168851975376981911770679412Obs.ArSpecics225422702285962154213405821006908823B2-spccics229322541530’ 13221301 625887833A2-spccics690798327Brspccics823762530567371Qalc.2|612|762|86963154413525911020873802229022551515132712656281010891835669787330826739555597371Obs.304830901310736159215465481074245998310716141284145556812331145346501876219993796720469Calc.3043307313037391572152653210672431008988308016371324145756012321142352500876228960277799718463Vibrationalassignment"100(2)99(13)98(20fl)79(1)71(8fl) + 25(9a) 66(18«)^^34(19«) 72(6fl)+ 13(9fl) 61(12)+ 15(18fl) 65(9fl) + 16(8 )^ 64(l) + 31(18fl)98(76)100(206)70(86)+ 30(186) 59(3) f 35(14) 40(196)4 59(3) 61(66)+ 19(86) 78(3)+ 20(14) 42(196)+ 35(15) 59(14)+ 27(186)lOO(lOfl)I00(17tf)98(16ti)100(5)100(106)100( 11)100(4)100(166)l^ csults in this column correspond to PYR. TTic results of other molecules can be obtained from the author. Number before the parenthesis is % PED and in the parenthesis is the mode in Wilson's notation [7]. observed.'■ Vibrational assignmentsassignments presented in Table 1 are self-explanatory.the discussion is confined to certain important modes'niy.C-C and C~N stretching vibrations: molecules described here, modes 8a and ib  are expected 'found 1600 cm"‘. The higher frequency has a large amountof C-C stretching character ranging from 70 to 84% in the three molecules investigated. As expected, it mixes with the C-H/C-D bending mode 9a and 9b in PYR and DFP, respectively, whereas it is pure in PYR-d5. The lower frequency is a C-C stretching mode to the extent of 70 to 80%. In PYR and PYR-d5, this mode mixes with C-H/C-D in-plane bending mode 18b, while in DFP it is pure. According to the calculations made here, the frequency of mode 8o is476 B Venkatram Reddy and G Ramona Raogreater than that of ib  in PYR and PYR-d5, whereas reverse is true in DFP. Thus the bands at 1583,1542 and 1592 cm * are ascribed to mode 8a, whereas those near 1572,1530 and 1614 cm"* are attributed to vibration 86 in PYR, PYR-d5 and DFP, respectively.The modes 19a and 196 are expected in the range 1400- 1500 cm~*. On the basis of calculations the bands at 1482, 1340 and 1546 cm"* are assigned to mode 19a, while those around 1439,1301 and 1455 cm * are attributed to mode 196 in PYR, PYR-d5 and DFP, repectively. The modes 19a and 196 mix with C-H/C-D in-plane bending modes 18a and 3, respectively in the three molecules.The mode 14, called the Kekule mode, has been assigited around 1375,1322 and 1284 cm * in PYR, PYR-d5 and DFP, respectively. This mode mixes with C-H/C-D in-plane bending vibrataion 3, 15 and 96 in PYR, PYR-d5 and DFP, respectively. It also mixes with C-F stretching mode 76 in DFP.The assignment given for the C-C stretches in pyridine on the basis of present calculations is in agreement with that given by Green et al [6], The modes 1 and 86 should be considered as the C N stretching vibrations on the basis of % PED.3.2. Ring vibrations:The ring vibrations 1, 6a, 66 and 12 are identified and assigned on the basis of their characteristic eigen vector distribution. Let us consider the modes 6a and 66 corresponding to the benzene band at 606 cm'*. According to normal coordinate analysis, the absorptions near 605, 582 and 548 cm"* are due to the mode 6a, whereas those around 652, 625 and 568 cm * are attributed to mode 66 in PYR, PYR-d5 and DFP, respectively. The mode 6a mixes with C-H /C-D  in-plane bending mode 9a in PYR and PYR-d5, whereas in DFP it mixes with C-C stretching mode 19a.The absorptions near 1068, 1006 and 1074 cm"' are attributed to vibration 12, whereas the fundamentals near 992, 962 and 736 cm"* are ascribed to mode 1 in PYR, PYR-d5 and DFP, respectively. The mode 12 mixes with C-H/C-D in-plane bending vibration in the three molecules, whereas mode 1 is pure in PYR and PYR-d5, while it mixes with mode 12 and C-F stretching vibration 20a in DFP. The assignment for modes 12 and 18a in pyridine given by Green et al [6] is reversed in the inesent calculations.3.3. C-H/C-D/C-F in-plane bending vibrations :The modes 3, 9a, 18a, 186 and 15 designate the O H  and O D  in-plane bending vibration, in PYR and PYR-d5, whereasin DFP the C-H in-plane bending modes are designated as 3 18a and 96. The highest O H /C -D  in-plane bending vibration is mode 3. The bands near 1288, 1010 (calc, value) and 1233 cm * are due to mode 3 in PYR, PYR-d5 and DFP respectively. This mode mixes with Kekule mode in pyp whereas in PYR-d5 it is pure, while in DFP it mixes wnj C-F stretching mode 76 and mode C-C stretching mode 19b.On the basis of calculations, the pair of bands at 1030 and 1148 cm'*, and 823 and 833 Cm * are attributed to modes I8a and 186. Similarly, the modes 9o and 15 are identified near 1218 and 1085 cm*' and 908 and 887 cm~*, rcspectivel> m PYR and PYR-d5. The bands near 1145 and 1008 cm '(calc value) are ascribed to C-H in-plane bending modes % and 18a in DFP. As is evident from Table 2, these modes mix with C-C stretching vibration 196 in PYR and PYR-d5, whereas in DFP the mode 96 mixes with C-C stretching vibration 196 and mode 18a mixes with C-F stretchini; vibration 20a.The pair of bands near 245 and 346 cm ' are assigned Ui C-F in-plane bending mode 9a and 186, respectively, in DFP The mode 9a mixes with C-C stretching mode 1, whereas tiic mode 186 mixes with mode 196.3.4. C-H/C-D/C-F out-of-plane bending vibrationsIn PYR and DFP, the number of C-H out-of-planc bendint; modes, known as C-H waggings is 5 and 3, respectivelv. whereas in PYR-d5 the number of C-D out-of-plane bending modes is 5. They are designated as 5, lOa, 106, 11 and 17tr m PYR and PYR-d5, while in DFP they are identified as 5,11 and 17a.The bands at 942, 823 and 993 cm ' are due to mode 5. whereas those at 749,530 and 796 cm"' are attributed to mode 11, while the absorptions near 891 and 690 cm"' are assigned to mode 10a, whereas mode 106 is identified near 886 and 762 cm ' in PYR, PYR-d5 and DFP respectively. It is interesting to note that all the five modes are pure in PYR and PYR-d5 except mode 11, which mixes with ring torsion 4 in DFP, the mode 11 mixes with C-F out-of-plane bending mode 106 and CCCC torsional mode 4, whereas the other two modes are pure.The two C-F wagging modes are designated as 10a and 106 and identified near 501 and 227 cm"* (calc, value) in DFP' respectively. The mode 10a is pure, whereas mode 106 mixes with ring torsion 166.3.5. Ring torsions:The CCCC out-of-plane bending modes 4, 16a and 16* are known as ring torsions. The mode 4 is near 700,567 andNormal coordinate treatment of some pyridines 477■’20 cm ' in PYR, PYR-d5 and DFP, respectively. Similarly, mode I6fl is ascribed to the bands at 375,327 and 219 cm whereas those due to mode 16A are identified around 405, 371 and 469 cm'', respectively In PYR, PYR-d5 and DFP. (I IS seen that the mode 4 is the highest ring torsion in the three molecules. It is pure in PYR, whereas in PYR-d5 and DFP it mixes with C-H/C-D out-of-plane bending mode i; The present calculations establish that th e  f re q u e n cy  of mode 16fl is less than that of 166 in the three molecules. The modes 16u and 166 have no mixing in PYR and PYR-d5, whereas it mixes with C-F out-of-plane bending mode lOu in DFP.Rcferf»c»$(11 D A !• S M urfm  anJ F I  1'hon.d.s Ti ms. F a ra d a v  5o r 59 12 (1963)[2] R T  Bailey and I)  Steele S p e c im c h im  ic  ta  23A 2997(1967)[31 P Muralidhar Rao and r i Ramana Rao J. R a m a n  S p e c ir o sc  20 529 (1989)(4) |h I  akshmaiah and G Ramana Rao J  R a m a n  S p e c t m c  20 439(1989)|5) j) M Schachtschneider Tech R ep o rt No. 57-65, Shell Development C o , Em eryw illc , C aliforn ia  (1966)[6] J n  S Green, \V Kynaslon and H M  Paisley S p ec tro ch im . A c ta  19 549 (I96J)171 E H  Wilson Plm Rev 45 706(1934)

Normal Coordinate Treatment of some Pyridines

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Normal Coordinate Treatment of some Pyridines

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