Vibrational spectra of monosubstituted pyridines

Spectrochlmlra Acta, 1963, Vol 19,~~. 549toX1. Pergamon Press Ltd.Printed inNorthern Ireland

Vibrational spectra of monosubstituted pyridines J. H. S. GREEN,

W.

and H. M. PAISLEY

KYNASTON

Kational Chemical Laboratory,

Teddington,

(Recezwed 14 July

Middlesex

1962)

Abstract-The mfra-red (4000-400 cm-l) and Raman spectra of 2-, 3- and 4-chloro and bromopyridmes have been measured. Complete assignments of the frequencies are proposed for these and the correspondmg methyl pyrldmes, and for the Raman displacements of 2- and 3-fluoropyndine. The assignments are correlated with one another and with those for the deutero compounds.

INTRODUCTION Sonl~ years ago the infra-red

spectra of pyridine and of 2-, 3- and 4-methyl pyridine were recorded in this laboratory and published [l] together with the Raman displacements. Measurements of both the infra-red and Raman spectra of 2-, 3- and 4-chloro and bromopyridine are now reported. Assignments of the observed frequencies and of the normal frequencies of these nine compounds, and for the Raman shifts [2] of 2- and 3-fluoropyridine, have been made and are correlated with one another and with those of the corresponding monodeuterated pyridines [3]. EXPERIMENTAL

The materials used were of commercial origin and varied considerably in purity. The samples were dried and fractionated under reduced pressure, the course of 4-Chloropurification being followed by measurement of the infra-red spectrum. pyridine could not be so purified because of excessive decomposition. Fractional freezing by D. HARROP gave a material whose spectrum was in good agreement with the partial data available [4, 51. The impurities in this detectable by gas chromatography by R. R. CJOLLERSON (stationary phase 10% apiezon L on celite at 1SO’C) were no more than 1 per cent. Infra-red and Raman spectra were recorded as described previously [6]. RESULTS

AND

ASSIGNMENTS

For the methyl, chloro- and bromo-pyridines the observed frequencies and their assignments are assembled in Tables 1-9; the values for the methyl compounds are from the previous measurements [l] supplemented by more recent A. LoNG,F. S.MURFIN,J. L. HALES and 11’. KYNASTON, Truns Faraday Sot. 53, 1171 (1957); E. HERZ,L. KAHOVEC and K. W.F. KOHLRAASCH, Z.physik.Chem. (Lepig) B53, 124 (1943). [2] H. P. STEPHENSON and F. L. VOELZ, J. Chem. Pkys. 22, 1945 (1954). [3] F. A. ANDERSON, B BAK, S. BRODERSEN and J. RASTRAP-ANDERSEN, J. Chew. Phys. 23, 1047 (1955). [4] Index of Infrared Spectra, No. 18955 Sadtler, Philadelphia. [5] G. COSTA and P. BLASINA, 2. pkysik. Chem. (Frunk&wt) 4, 24 (1955). [6] .J. H. S. GREEN, J. Ckem. Sot. 2236 (1961); Spectrochim. Acta 1’7, 607 (1961). [l]I).

549

J. H. S. GREEN, W. KYNASTON and H. M. PAISLEY

550

Table 1. Vibrational

Infrared

Raman Av (liquid) [I] 211(2) 341(l) 384(O) 485( 1) 514(4) 669( 4)

frequencies and assignment for 4-methylpyridme

p

Liquid

Vapour [l]

0.9 O-8 -0.7 0.31 0.73

490(s) 523(m) 668(w) 728(s) 799(vs)

801(8) 866( 00) 937(00) 969(O)

0.21

994( 10)

0.21

727 c 793 800 8071

872(w) 945(w) 972(m) 994(s)

990 996 10011

Interpretation 6, fundamental b, fundamental rss fundamental b, fundamental a1 fundamental b, fundamental b, fundament&l b, fundamental a1 fundamental aa and b, fundamentals 728 + 211 = 939(A,) ~.s fundamental al fundamental CH, rock

1042(s) 1061 1068( 1)

0.42

1070(m) 10711 1080

872 $- 211 = 1083(A,)

1090(w)

1212(s) 1223(s) 1289(w) 1340(w)

1100I 1113 1157 1216 1228 1279 1344

1365(m)

1356

1114(w) 1148(O) 1212(3) 1220(5) 1 1282(O) 1329(O)

1378( 1) 1409f 1) 1449(O) 1495( 1) 1561( 1) 1603(4)

0.32

P

1383(s) P

P dp?

1417(vs) 1445(vs) 1495(s) 1566(s) 1604(vs) 1669(m) 1755(n) 1795(w) 1854(w) 1935(w) 2910(s)

2934(5) 2959(l) 2983(2) 3029(3) 3050( 10) (w)

= weak;

2970(s) 3010(s) 3040(s)

(m) = medium;

b, fundamental

1420 1501 1575 1603 1660 1744

1846 1927 2884 2937 3008 3038 3070 (R) = strong:

(v) y very:

384 + 728 = 1112(B) 211 + 945 = 1156(A,) a1 fundamental al fundamental b, fundamental 2 x 668 = 1336(x4,); 994 + 341 = 1335(B) 972 + 384 = 1356(A,); 485 + 872 = 1357(A,) C II, sym. deformation 341 i 1068 = 1409(A,) 6, fundamental CH, antisym. deformation a1 fundamental b, fundamental a1 f~dament,al 872 + 799 = 1671(A,) 945 + 799 = 1744(B); 2 x 872 = 1744(x4,) 799 f 994 = 1793(B) a72 -f- 972 = 1844 2 k 972 = 1944(x4,) CH, sym. str. CH, antisym. str. b, fundamental al and b, fundamentals a1 fundamental (sh) = shoulder.

Vibrational Table 2. Vibrational Raman 182(s) 300(w) 39O(vw) 415(s) 495(w) 663(s) 712(s) 813(vw) 892(w) 989(s) 996(vs) 1064(m) 1103(s) 1133(m) 1219(s)

1322(vw)

1412

1516(vw) 1572(s)

3048(s) 3076(s)

Infra-red

412(m) 491(s) 662(w) ,69O(sh) 712(vs) 722(sh) Sll(vs) 856(w) 914(w) 955(m) 984(m) 995(m) 1063(s) 1080(w) 1103(vs) 1131(s) 1212(s) 1219(s) 1306(sh) 1316(m) 1359(m) 1377(m) 1407(s) 1449(m) 1484(s) 1516(w) 1566(s) 1575(s) 1621(w) 1665(w) 1763(w) 1806(w) 1836(w) 1933(m) 3050(s) 3074(s)

spectra of monosubstituted

pyridines

551

frequencies and assignment for 4-chloropyridine Interpretation 6, fundamental b, fundamental a2 fundamental al fundamental b, fundamental b, fundamental 182 + 495 = 677(A,); 300 + 390 = 69O(B,) al fundamental b, fundamental b, fundamental 6, fundamental a2 fundamental a2 fundamental 2 x 491 = 982(A,) a1 fundamental a1 fundamental b, fundamental ai fundamental 712 + 415 = 1127(A,) 722 + 491 = 1213(A,) a, fundamental 995 + 300 = 1295(B,); 390 + 914 = 1304(A,); 811 + 491 = 1302(A,) b, fundamental b, fundamental 712 + 662 = 1374(B,) b, fundamental 2 x 722 = 1444(A,) ai fundamental 412 + 1103 = 1515(A,) b, fundamental ai fundamental 2 Y 811 = 1622(A,) 856 + 811 = 1667(A,) 955 + 811 = 1766(B,) 996 + 811 = 1807(B,) 2 x 914 = 1828(/l,); 955 + 856 = 1811(B,) 2 x 955 = 1910(d,) a1 and b, fundamentals a, and b, fundamentals

infra-red spectra [7].Agreement with partial infra-red measurements for all the halogenopyridines [8] and with the previously recorded Raman spectrum of 2dichloropyridine [9]is good. [7] Catalogue of Infrared Spectra, Nos. 2204, 2205, 2190, 2191. American Petroleum Institute Research Project 44 (1960). Catalogue of Infrared Spectra, Nos. l-4. Manufacturing Chemists Association Research Project (1959). [S] Index of Infrared Spectra, Nos. 1795,4728,16341,17133,18953,18955, Sadtler, Philadelphia. [9] J. W. MURRAY and D. H. ANDREWS, J. Chem. Phya. 1,406 (1933).

J. H. S. GREEN, \V. KYNAYTON and H. M. PAISLEY

552

Table 3. Vibrational Reman 182(s) 256(w) 317(vs) 484(w) 662(s) 680(s) 723(vw) 802(vw) 896(vw)

1063(s) 1094(vs) 1220(s) 1320(w) 1408(w) 1480(w) 1566(s)

3048(s) 3076(w)

frequencies and assignment for 4-bromopyndme

Infra-red

Int,erpretation

482(s) 662(sh) 679(vs) 722(m) 805(vs) 859(s) 900(w) 961(s) 992(s) 1062(vs) 1076(m) 1091(vs) 1115(w) 1216(vs) 1291(w) 1316(s) 1229(w) 1403(vs) 1435(w) 1482(vs) 1567(vs) 1608(w) 1659(m) 1766(w) 1827(w) 1925(m) 3035(s) 307 2(s)

b, fundamental b, fundamental a1 fundamental b, fundamental b, fundamental al fundamental 6, fundamental b, fundamental b, fundamental 722 + 182 = 904(A,) a2 fundamental al fundamental al fundamental 6, fundamental a1 fundamental 805 + 317 = 1122(B,); 859 + 256 = 1115(_4,) al fundamental 805 + 482 = 1287(A,) b, fundamental b, fundamental b, fundamental 2 x 722 = 1444(/l,) a, fundamental a, and b, fundamentals 2 x 805 = 1610(/l,) 805 + 859 = 1664(A,) 961 + 805 = 1776(B,) 2 x 914 = 1828(A,); 961 + 859 = 1820(23,) 2 x 961 = 1922(A,) a1 and b, fundamentals al and b, fundamentals

Pyridine The vibrational spectrum of pyridine was last considered by WILMSHURST and [lo] whose notation for the fundamentals is used here apart, from an interchange of the b, and b, species to conform to the recommended [ 1 l] choice of axes for molecules of C,, symmetry. The a, class is satisfactorily established, but two comments are called for concerning the 6, class. Firstly, the quoted [ll] frequency at 1085 cm-l has been reported [l] as a weak infra-red band; secondly, it, is preferred here to place Ye at 1290 cm-l rather than to use a second time [lo] the band at 1218 cm-l. A weak band at 1293 cm-l is observed in the infra-red spectrum of liquid pyridine (1290 cm-l in the vapour [l]) and the frequency appears at 1288 cm-l as a Raman shift of medium intensity in the photoelectrically recorded spectrum. BERNSTEIN

[lo] J. K. VC’ILMSRURST and H. J. BERNSTEIN, Calz. J. Chem. 35, 1183 (1957). [ll] R. S. MULLIKEN, J. Chem. Phys. 23, 1997 (1933).

Vibrational Table

4. Vibrational

Raman Av(liquid)

,Z [l]

207(3) 0 65 360(O) 403(00) 470(O) 545(4) 0.43 629(2) 0.71 709(00) 726(O)

Liqmd

729(s) 751(w)

810(w) 553(w) 940(w) 972(m)

700 732 747 C 755 I 799 503 509 I 520 925 972

957(l) 994(O) 995(10) 0.07

994(s)

1036(O) 1050(10) 0.05 1100(l) dp? 1149(l) 0.75

1040(s) 1047(s) 1099(m) 1143(s)

1236(7) 0.14

1233(m)

and assignment

954 999 A 1004I 1047 1056 1095 1142 1155 1 1241

553

pyridmes for 2-methyl pyridme

Interpretation

[l]

355(s) 403(s) 470(s) 545(m) 625(m)

0 11 512(3) 1 553(O)

of monosubstituted

frequencies

Infra-red Vapour

795(m)

SOO(7)

spectra

n” fundamental o’ fundamental o” fundamental n” fundamental a’ fundamental n’ fundamental 1046 - 355 = 655(A’) a” fundamental (I” fundamental

a’ fundamental 2 x 403 = 506(A’) a” fundamental o” fundamental a” fundamental 360 + 629 = 989(x4’)

s’ fundamental CH, rock a’ fundamental a’ fundamental a’ fundamental rc’ fundamental

1251 1 403 +

1254(O) 1295(2) 0.54

1291(s)

1376(4) 0.41

1376(m)

1425( 1) dp? 1459(O) 1479(O) 1565(4) 0.77 1559(3) I

1440(w)

1291 1299 13051 1351

1286(A’)

(I’ fundamental

n’ fundamental 1359I 1436

o’ 2 n’ n’

and CH, sym. deformation

fundamental and CH, antisym. x 729 = 1455(A’) fundamental fundamental

1475(w) 1565(w)

1465

159O(vs)

1594

n’ fundamental

1773(w) 1845(w) 1576(w) 2990 2950(s)

1763 1539 1564 2939

2 x 2 x 972 2 x CH, CH,

3030(s)

3027

3050(s)

3052

1591(3) I 1600(3)

2925(S) 2960( 1) 2955(O) 3012(l) 3046( 10) 3059(b)

553 =

deformation

799 = 1595(A’); 553 -i_ 729 = 553 = 1766(A’) + 553 = 1555(A’) 940 = 1556(A’) sym. str. antwym. str.

b2 fundamental a, and b, fundamentals a, fundamental

1612(/i’)

J. H. S. GREEN, W. KYNASTON and H. M. PAISLEY

554

Table 5. Vibrational Raman

P

19O(vs) 313(m)

dp

428(vs) 481(w) 621(s) 728(s) 772(w) 821(vw) 889(m) 967(w) 995(vs) 1045(vs) 1085(s) 1121(s) 1153(s)

0.5

1242(m) 1291(s) 1367(w) 1423(m) 1459(m) 1482(vw) 1549(vw) 1573(s) 1582(s)

3058(vs) 3071(s)

P? 0.33

P

frequencies and assignment for 2-chloropyridine

Infrared

406(w) 425(m) 480(m) 617(m) 725(s) 767(s) 827(vw) 881(w) 960(w) 992(s) 1044(s) 1083(s) 1117(vs) 1148(s) 1153(sh) 1236(vw) 1286(m) 1363(w) 1420(s) 1452(s) 1467(sh)

1568(s) 1577(s) 1610(m) 1651(m) 1663(m) 1732(w) 1763(w) 1838(m) 1870(m) 1920(w) 1955(m) 1983 3059(s) 3082(sh)

Interpretation a” fundamental a’ fundamental a” fundamental a’ fundamental a” fundamental a’ fundamental a’ and u” fundamentals a ” fundamental 406 + 425 = 831(/l”) a” fundamental a" fundamental a’ fundamental a’ fundamental a’ fundamental a’ fundamental a’ fundamental 725 + 425 = 1150(A’ord”); 960 + 190 = 1150(A’) 2 x 617 = 1234(A’) a’ fundamental a’ fundamental a’ fundamental a’ fundamental 2 x 725 = 1450(x4’) 725 + 767 = 1492(A”) 2 x 767 = 1534(/l’) a’ fundamental a’ fundamental 882 + 722 = 1603(A’); 617 + 992 = 1609(/l’) 722 + [935] = 1657(A’) 722 + 960 = 168(A’) 960 + 767 = 1727(/l’) 2 x 881 = 1762(A’) 960 + 881 = 1841(A’) 2 x [935] = 1870(/l’) 2 x 960 = 1920(A’) 960 + 992 = 1952(A”) 2 x 992 = l984(A’) 4~’ fundamentals

Previous attributions have been to 2~~~ (A,) or Ye, + yI1 (I?,), both of which seem too high in value. Another alternative is ~r,,~+ vleb = 1291 (A,) but this is not available for the substituted pyridines. The proposed change brings the value for ys into line with that for benzene and monosubstituted benzenes. With it, the product-rule ratio for pyridine and pyridine-d, [3] is still satisfied, provided 908 cm-l is replaced by 962 cm-l as the corresponding frequency in the latter compound, where it is therefore taken as an a, and a b, mode. The weak infra-red band at 908 cm-r is then interpreted as 371 + 530 = 901 (A,). The out-of-plane classes of pyridine have been treated in detail by KOVNER

Vibrational Table 6. Vlbratlonal

-_--

Raman __176(vs) 264(m) 315(vs) 409(w) 47O(vw) 615(m) 703(s) 77O(vw) 888(w)

992(vs)

1044(vs) 1078(m) 1106(m) 1147(m) 1235(m) 1283(m) 1416(w) 1450(m) 1561(s) 1569(s)

~~~__.

spectra of monosubstltuted

frequencies and assignment for 2-bromopyridme Interpretation

Infra-red

-___

-~~-

404(w) 466(s) 613(m) 700(s) 724(w) 761(vs) 882(m) 934(m) 960(w) 977(w) 989(m) 1016(w) 1042(w) 1076(s) 1104(s) 1146(m) 1239(w) 1282(m) 1363(w) 1417(s) 1452(s) 1563(s) 1573(s) 1603(m) 1603(m)

3055(s) 3069(s)

1644(w) 1667(w) 172O(vw) 1761(vw) 1841(w) 1874(vw) 1915(vw) 1952(w) 1977(vw) 3056(s) -

565

pyrldmes

a” fundamental a’ fundamental a’ fundamental u” fundamental a” fundamental a’ fundamental a’ fundamental CA”fundamental a” fundamental a” fundamental a” fundamental a” fundamental 700 + 264 = 964(A’) a’ fundamental 761 + 164 = 1025(A”); 700 + 315 = 1015(x4’) 713 + 404 = 1017(A”) a’ fundamental a’ fundamental a’ fundamental a’ fundamental 2 x 613 = 1226(x4’); 761 + 466 = 1227(A’) a’ fundamental a’ fundamental a’ fundamental a’ fundamental a’ fundamental a’ fundamental 1281 + 315 = 1592(A’); 613 + 989 = 1602(./l’): 882 + 724 = 1606(A’) 1282 + 315 = 1597(/i’); 613 + 989 = 1602(/l’); 882 + 724 = 1606(A’) 882 + 761 = 1643(A’) 724 + 960 = 1684(/l’) 960 + 761 = 1721(/l’) 2 x 882 = 1764(/I’) 960 + 882 = 1842(A’) 2 x 934 = 1868(A’) 2 x 960 = 1920(A’) 960 + 989 = 1949(A’) 2 x 989 = 1978(A’) I

4a’ fundamentals

et al. [12].

This work led to assignments for pyridine in agreement with those of WILMSHURST and BERNSTEIN [lo],apart from the change of vlobfrom 886 to 981 cm-l made arbitrarily by the latter authors to fit thermodynamic data; the choice of 886 cm-l is retained here.

[12] M. A. KOVNER, Yu.

(1961).

S. KOVOSTELEV and V. I. BEREZIN, Optzka i Spektroskopiya

10,457

J. H. S. GREEN,

556

Table 7 Raman Av (liquid) [ 11 217(4) 338(2) 398( 1) 454( 1) 535(5) 575(00) 628(3)

Vibrational

630(m)

795(5)

788(W)

810(5) 0.16 919(00) 942(00) 984(O) 1025(5) 0.13 1041(10) 1046(O)

923(w) 941(w) 987(m) 1028(s) 1043(s)

1103(O) p?

1106(s)

1 l24(0) 1153(O) dp?

1125(m) 1165(w)

1190(5)

1188(s)

1227(5) 0.28?

1227(m) 1247(w)

1340(m) 1385(s) 1414(W) 1452(vs) 1479(W) 1578(vs) 1595(s) 1659(m) 1720(m) 1765(w) 1838(w) 1875(w) 1910(w) 1969(w) 2895(s)

Interpretation a” fundamental a’ fundamental a” fundamental a” fundamental a’ fundamental 788 - 217 = 571(x4’) a’ fundamental

338(w) 399(m) 457(m) 538(m)

708(W)

2923(3) 2941(l) 2975(O) 2995( 1) 3030(3) 3054(5) 308.5( 1)

frequencies end assignment for 3-methylpyridme

Infra-red Liquid Vapour [l]

710(O)

1262(O) 1286(O) 1336(O) 1380(3) 1409(5) 1453(O) 1477(l) 1575(3) 1594(6)

TV. KYNASTON and H. M. PAISLEY

702 711 c 718 i 776 782 C 790 I 920 984 1023 1035 1097 1111 1119 1 1130

1186 1193 1198 1 1231

1333 1381 1419 1472 1581

1708 1852 1904

2935

a” fundamental

U” fundamental a’ fundamental a” fundamental a” fundamental a” fundamental a’ fundamental A’ fundamental and CH, rock 708 + 338 = 1046(A”) a’ fundamental 338 + 788 = 1126(.4’) 941 + 217 = 1158(A’); 708 + 457 = 1165(A’); 630 + 538 = 1168(A’) a’ fundamental a’ fundamental 217 + 1028 = 1245(x4”); 708 + 538 = 1246(A”) 2 x 630 = 1260(x4’) a’ fundamental a’ fundamental CH, sym. deformation a’ fundamental CH, antisym. deformation a’ fundamental a’ fundamental a’ fundamental 708 + 941 = 1649(A’) 923 + 788 = 1711(/l’) 788 + 987 = 1775(A’) 2 x 923 = 1846(A’) 2 x 941 = 1882(A’) 987 + 923 = 1910(A’) 2 x 987 = 1974(A’) 2 x 1495 = 299O(A’) CH, sym. stretch CH, entisym.

2980(s) 3015 3040

stretch

4~’ fundamentals

Vibrational Table 8. Vibrational Raman 199(vs) 294(m) 405(vw) 428(s) 463(vw) 619(m) 707(vw) 731(s) 803(w) 939(w) 1041(vs) 1097(s) 1107(s) 1155(m) 1192(s) 1230(m) 1321(vw) 1372(vw) 1416(w) 1468(m) 1569(s) 1573(s)

3052(vs) 3079(s)

spectra of monosubstituted

557

pyridmes

frequencies and assignment for 3chloropyridine Interpretation

Infrared ___~

403(m) 426(s) 461(w) 615(s) 7OO(vs) 728(vs) 796(vs) 915(m) 943(m) 1016(vs) 1034(m) 1095(vs) 1106(vs) 1118(sh) 1157(m) 1190(m) 1227(w) 1319(m) 1385(sh) 1417(vs) 1469(vs) 1569(s) 1573(s) 1613(w) 1630(w) 1651(w) 1713(m) 1738(m) 1826(m) 1854(m) 1891(m) 1920(m) 1958(w) 3052(s)

a” fundamental a’ fundamental a” fundamental a’ fundamental a” fundamental a’ fundamental a” fundamental a’ fundamental a” fundamental a” fundamental a” fundamental a’ fundamental a’ fundamental a’ fundamental a’ fundamental 403 + 700 = 1103(A’); 915 + 199 = 1114(A); 728 + 426 = 1154(A’); 700 + 461 = 1161(A’) a’ fundamental a’ fundamental a’ fundamental 2 x 700 = 1400(A’); 199 + 1190 = 1398(A”); 294 + 1095 = 1389(/l’) a’ fundamental a’ fundamental a’ fundamental a’ fundamental 915 + 700 = 1615(./l’) 943 + 700 = 1643(x4’) 1041 + 615 = 1656(A’) 795 + 915 = 1710(A’) 1319 + 426 = 1745(A’) 2 x 915 = 1830(A’) 915 + 943 = 1859(x4’) 2 x 943 = 1886(A’) 980 + 915 = 1895(A’) 943 + 980 = 1923(A’) 2 A 980 = 1960(A’) 4~’ fundamentals 1

The assignment for pyridine which formed the basis of the present work is summarized in Table 10 together with approximate description of the modes. Approximate normal co-ordinate calculations [13] show that there is appreciable mixing of C-C stretching and C-H bending in most of the vibrations. There is a small contribution of C-N stretch to us, whilst ~rs,, vlsb and q4 all involve C-H bending and the modes designated p(CH) all involve ring deformation. [13] E.

TVACHSMANN

and E. W.

SCHMIDT,

2.

physk

Chem. (Fnznkfurt)

27, 146 (1961).

558

J. H. S. GREEN, W. KPNASTON and H. M. PAISLEY Table 9. Raman 182(s) 246(m) 319(s) 358(vw) 401(w) 451(w) 506(vw) 614(w) 705(m) 797(vw) 936(w)

1035(vs) 1087(m) 1117(vw) 1191(m) 1225(w) 1287(vw) 1325(vw) 1416(w) 1468(w) 1558(s) 1573(sh)

3050(s)

Vibrational frequencies and assignment for 3-bromopyndine Infra-red

447(w) 612(m) 699(vs) 792(vs) 915(m) 944(m) 978(sh) 1008(vs) 1024(w) 1034(vw) 1085(s) 1094(s) 1115(m) 1189(m) 1221(vw) 1282(vw) 1320(m) 1415(vs) 1434(sh) 1467(vs) 1559(s) 1573(s) 1643(w) 1710(w) 1731(m) 1826(w) 1854(m) 1895(m) 1920(m) 1958(w) 3052(s) 3082(sh)

______

Interpretation --~

--

a” fundamental a’ fundamental a’ fundamental 2 x 182 = 364(A’) a” fundamental u” fundamental 2 x 246 = 492(A’) a' fundamental a’ and a# fundamentals a” fundamental a” fundamental a” fundamental a” fundamental a’ fundamental a’ fundamental 792 + 246 = 1038(A”) a’ fundamental u’ fundamental 792 + 319 = 1111(A”); 699 + 401 = llOO(d’or A”) a’ fundamental a’ fundamental 1094 + 182 = 1276(x4”) a’ fundamental a’ fundamental 1189 + 246 = 1435(d’) a’ fundamental a’ fundamental a’ fundamental 700 + 944 = 1644(/t’); 1035 + 614 = 1649(d ) 792 + 915 = 1707(A’) 1035 + 705 = 1742(/A’); 1416 + 319 = 1735(A’) 2 x 915 = 1830(A’) 915 + 944 = 1859(A’) 2 x 944 = 1888(A’); 978 + 915 = 1893(4’) 978 + 944 = 1922(A’) 2 x 978 = 1946(A’) 4~’ fundamentals

4-Substituted pyridines. The 4-substituted pyridines, having the highest symmetry, are considered first,thevibrations comprising lOa, + 9b, + 3a, + 5b, modes. The internal methyl vibrations are discussed separately. The C-H stretching frequencies are not all resolved and the distribution of the observed frequencies between 2a, and 2b, modes is rather arbitrary. The two pairs of essentially C-C and C-N stretching modes, with one member in each of the a, and 6, classes are obvious as strong infra-red bands; only in 4-bromopyridine are vs, and vsb unresolved. Since vlsbinvolves some C-X and C-H bending it is found appreciably The remaining aI modes are readily (about 75 cm-l) lower than its a, counterpart.

Vibrationel Table 10.

v(CH) v(CH) v(CC) v(CC, CN) B(CH) B(CH) Rmg X sens. X sens X sens. v(CH) v(CH) v(CC) v(CC, CN) v(CC, CN) B(CH) B(CH) a(CCC) X sens. SW YK’H) 4Ku NH) YWW 4v3 WC)

X sens.

spectra of monosubstituted

pyridines

559

Fundamental vibrational frequencies of pyridine and 4-substituted pyridines

2 20a 8a 19a 9a 18a 1 13 12 6a 20b 7b 8b 19b 14 3 18h 6b 15 17a 1oa 16a 5 lob 4 11 166

PY

4-DPy

4-MePy

4-ClPy

4-BrPy

3054 3054 1583 1482 1218 1068 992 3036 1030 605 3080 3036 1572 1439 1375 1288 1085 652 1148 986 891 375 942 886 749 700 405

3033 3019 1575 1476 1215 1068 989 2285 1010 597 3069 3035 1559 1413 1374 1269 1086 648 862 979 889 373 890 834 743 625 371

3050 3040 1604 1495 1220 1070 994 1212 801 514 3040 3010 1566 1417 1365 1289 1090 669 341 972 872 384 872 799 728 490 211

3076 3048 1575 1484 1219 1064 996 1103 712 414 3076 3048 1564 1407 1359 1316 1080 663 300 955 914 390 836 811 722 491 182

3072 3035 1567 1482 1216 1062 992 1091 680 317 3072 3035 1566 1403 1339 1316 1076 662 256 961 914

WOI 859 805 722 482 182

identified as strong Raman lines; three X-sensitive vibrations arise as in the corresponding monosubstituted benzenes and the values found here are very close to In the al class, the previous assignment for 4-deuterothose for these compounds. pyridine [3] is entirely in accord with the present work. In the b, class the assignments for y14and vS follow from the present values for pyridine. For &deuteropyridine, v14 may be brought into line with the other compounds by use of the 1374 cm-l band which is slightly stronger than the 1347 cm-l band taken previously [3]. The latter may be vlaa + v~,~ = 1352 (A,). Neither of the frequencies available for vg, 1237 and 1269 cm-l, entirely fit the pattern of the other compounds, but the higher is preferable, although it is the weaker. These choices lead t,o a product rule ratio for pyridine and C-deuteropyridine of O-725 in the b, class; the theoretical value is 0.724. The essentially /?(CH) mode vlBb is expected at ca. 1080 cm-l from its position in pyridine and it appears only weakly in the infra-red spectrum. The planar ring deformation vsb at ca. 660 cm-l is found especially strong in the Raman spectrum and only weakly in the infra-red. Only one b, mode is sensitive to the mass of the substituent and is found at values very close to those for the corresponding benzene compound. For the out-of-plane classes, the calculations of KOVNER et al. [12] provide

J. H. 8. GREEN, W. KYNASTON and H. IM. PAISLEY

560

values for 4-deuteropyridine and hence for the other compounds; the a2 class is virtually unchanged throughout the series. The two highest b, frequencies are y(CH) modes and the lower, at around 800 cm-l and intensely strong in the infra-red, is obviously the umbrella mode vloa. Its variation along the series 4-Me-, a-Cl-, 4-Brpyridine (799, 811 and 805 cm-l) closely parallels that for the corresponding psubstituted toluenes (794, 806 and 801 cm-l respectively). Two out-of-plane modes Table 11. Fundamental

a' al

4CW 4CW 4CC) v(CC, CN)

/WH) NW

Ring X sens. X sens. X sens. b2

v(CH) v(CH) v(CC)

v(CC,CN) v(CC, CN) B(CW /VW a(CCC) X sens. a” a 2

b,

YCH) y(CH) wc) I@H) NH) ww #CC) X sens.

2 20a 8a 19a 9a 18a 1 13 12 6a 20b 7b 8b 19b 14 3 18b 6b 15 17a 10a 16U 5 lob 4 11 16b

vibrational frequencies of 2-substituted

pyrldines

2-Dl’y

2-MePy

2-FPy

2-ClPy

2-BrPy

3051 3040 1576 1462 1149 1059 989 2258 1029 600 3075 3068 1570 1424 1378 1294 1112 640 834 970

3080 3046 1590 1475 1143 1047 994 1233 800 548 3080 3046 1565 1440 1376 1291 1099 629 359 972 883 403 940 751 729 470 207

3097 3074 1598 -

3080 3057 1577 1452 1150 1048 994 1120 727 428 3080 3057 1568 1420 1363 1288 1083 620 313 960 881 406

3069 3056 1573 1452 1146 1041 991 1104 701 315 3069 3056 1565 1417 1253 1282 1079 615 265 960 882 404 934 761 722 457 178

(886) 360 (932) 814 748 600 405

1146 1045 996 1249 828 556 3094 3074 1580 1303 1099 622

230

(935) 763 724 480 190

are found at ca. 725 and ca.490 cm-l for the heavier substituents and the lowest, essentially r(CX), vibration is observed as a medium to strong Raman live. The pairs of bands in 4-chloropyridine at 984, 995; 1103, 1131; and 1212, 1219 cm-l require a comment. These features are not due to impurities and are observable with the same relative intensities in previously recorded spectra 14, 51. They evidently arise from transitions to A, levels very close to the totally symmetric fundamentals; the relevant combinations are given in Table 2. 2- and 3-substituted pyridines. For 2- and 3-substituted pyridines the symmetry is lowered to that of the point group G, only, with 19a’ + 8~” vibrations, and the assignments in Tables 4-9 are made on that basis. A comparison with the corresponding frequencies of the 4-substituted compounds is at least formally possible and is given in Tables 10 and 11, but it must be remembered that the reduced symThus, there will metry results in a much greater mixing of the normal coordinates.

Vibrational

spectra of monosubstltuted

pyridines

561

of C-H deformation and C-X stretching in both of the adjacent this is reflected in the frequencies vlsb and y13in 2- and 3-chloro and bromopyridine; greater intensity of vlea compared with the 4-substituted compounds. In the planar class for 2- and 3-deuteropyridine the previous assignments are retained apart from changes of y14from 1357 and 1337 to 1378 and 1390 cm-l respectively, and of va from 1212 and 1217 to 1294 and 1262 cm-l, respectively. The assignments for the out-of-planar class are derived from those of KOVNER et al. [la]. Assignments of the observed [2] Raman frequencies for 2- and 3-fluoropyridine may be readily made in the light of the present work and are given in Tables 10 and 11. be contributions

Table 12. Fundamental

a’ al

VP-=) v(CH) VP3 v(CC, CN)

IWW WW

Ring X sens. X sens. X sens. b*

v(CH) v(CH) v(CC) v(CC, CN) v(CC, CN)

b2

20b 7b 8b

B(CH) B(CH)

19b 14 3 18b

Y(CH) y(CW ?wC) Y(CH) SW ww WC) X sens.

15 17a 10a 16a 5 lob 4 11 16b

a(CCC) X sens.

a” a2

2 20a 8a 19a 9a 18a 1 13 12 6a

6b

vibrational frequencies of 3-substituted

pyridines

3-DPy

3-MePy

3-FPy

3-ClPy

3-BrPy

3053 3038 1570 1476 1195 1050 980 3389 1033 599 3077 3030 1570 1416 1390 1262 1108 650 848 965

3085 3054 1594 1477 1190 1041 1025 1227 800 538 3054 3030 1575 1414 1340 1286 1106 628 338 987 923 399 941 788 708 457 217

3069 3058 1594 1480 1187 1038 1023 1247 818 535 3069 3058 1584 1425 -

3079 3052 1573 1469 1190 1040 1016 1096 730 428 3079 3052 1569 1417 1319 1227 1107 615 294

3082 3052 1573 1467 1189 1024 1008 1087 705 319 3082 3052 1559 1415 1320 1221 1094 614 246 978 915 401 944 792 699 447 182

(885) 355 938 823 717 404 632

1308 1095 616 982 410

702 244

[9801 915 404 943 795 700 460 199

In all the methyl pyridines the internal methyl group frequencies can be readily assigned. The symmetric and antisymmetric stretches are at 2924 (Raman value) and ca 2970 cm-l respectively, whilst the symmetric and antisymmetric bending modes are at 1380 and 1450 cm-l respectively; in 2-methyl pyridine the latter are coincident with a’ fundamentals. One methyl rocking mode is certainly at 1040 cm-1 (coincident with a’ fundamental in 3-methyl pyridine) and although there are other, weak, bands available as candidates for the second rocking mode, it is likely that it also is at 1040 cm-l. 11

J.

562

H. S. GREEN,W. KYNASTON and H. M. PAISLEZ' DISCUSSION

The characteristic frequencies of substituted pyridines have been discussed by several authors [14-181; most of this work is confirmed by the present results but some revisions and extensions are now possible. COOK and CHUBCH[14], using data for alkyl pyridines only, considered that the position and intensity of the combination bands in the region 1600-2000 cm-l were Table 13. Analysis of combmatlon

bands for 4-substituted D

___-

Me

pyridines

Cl

Br

__-

2 x lOb(A,) 5 + 106(A,) 17u + lOb(B,) 1Ob + l(B,) 17a + 5(B,); 2 x 17a(A,)

1724 1813 1861

2 x lOa

-

194x

1622

1610

lG71

1667

1654

1771 1795 1854

1766 1806 1828

1756 1827

1935

1933

1922

Table 14. Analysis of combination bands for 2-substituted pyrldines

10a + 4(A’) 10a + lOb(A’) 4 + 4 + 17a 2 x 17a 2 x 2 x 17a 2 Y

5W’) 17&t’) + lOb(d’) lOa + lOa 5(A’) 17cg.4’) + l(A”) l(A’)

D*

Me

Cl

Br

-

1600

1610

1603 1644 1655 1667 1720 1761

1716

1798 1783 1867 1894

1776 1845 1876

1960

1953

1987

-

1976

1651 1663 1732 1763 1838 1870 1920

1955 1983

1841 1874 1915

1952 1977

* An additional band at 1927 cm-l 1s probably (17~ + 5).

characteristic of the position of substitution. A detailed analysis of the possible origins of these bands can now be made and is given in Tables 13-15 which list the proposed interpretations; the strong bands are italicized. The same pattern of combinations arises in a regular way throughout each group of compounds. In particular, many of the bands clearly arise from out-of-plane CH bending fundamentals, as is expected. For the 1400-1600 cm-l region, the generalization has been made [14] that the separation of the higher pair of bands (us, and vsb of the present work) is about [14] G. L. COOK and F. M. CHURCH, J. Phys. Ghem 61, 458 (1957). [15] A. R. KATRITZKY and A. R. HANDS, J. Chem. Sot. 2202 (1958). [16]A.R. KATRITZKY, A.R.HANDs and R. A.JONES, J.Chem.Soc. 3165 (1958). [17] A. R. KATRITZKY and J. N. GARDNER, J. Chem. Sot. 2198 (1958). [18] H. SHINDO, Phurm. Bull. (Jupulz) 5, 472 (1957).

Vibrational

spectra of monosubstltuted

563

pyridmes

20 cm-l for 2- and 3-substituents, but about 40 cm-l for 4-substituents. In the light of the present work this appears to be true only for alkyl substituents and not The lower band, in the region 1403-1424 cm-l, whose for the halogenopyridines. origin was previously not apparent [14] is, in fact, quite clearly vlgb. For the lOOO1200 cm-l region, the suggested characteristic frequencies all involving CH bending correspond to the fundamentals assigned in Tables 10-12. The frequencies in the 600-800 cm-l region can be readily sorted by a comparison of the Raman and infra-red spectra. The intense infra-red band, vlOb is found at cit. 755, 790 and 800 cm-l for 2-, 3- and 4-substitution, respectively, the Table 15. Analpsls of combmatlon

bands for Q-substituted pyridmcs Me

4 + lOa(Ll’) 4 + 5(A’) 1% + 6b(d’) IOU + lOb(A’) 14 + 6a(A’)* 18a + 12; 19b + 6a(A’)* IOb + 17&l’) 2 x lOa 100, + k&4’) 2 x 5(d’) 1orc + 17a(A’) 5 + 170(_4’) 2 >: 17a(A’)

163’

1785 1760

1869

1896 1958

1659 1720 1765 1838 1875 1910 1969

Cl 1613 1630 1651 171.3 1738 -

Br

1643

1826 1852 1891

1826 1854 1895

1920 1958

1920 1958

* These involve X-sensitive fundamentals. t Addltlonal bands at 1949 and 1976 cm-I are probably (15 + 18b) and (14 + 6b) respect’ively.

trend being parallel to that for o-, m- and p-disubstituted benzenes. For the 3- and 4-methyl compounds, vlob is almost coincident with the strongly Raman active Xsensitive vibration vlZ. The latter moves to lower frequencies for the heavier substituents and becomes coincident with the essentially out-of-plane ring mode vq in, for example, P-chloropyridine. In 2-bromopyridine v12has moved still lower and vq is exposed as a weak band in the infra-red spectrum. In general, vq is rather more intense in the 3-substituted compounds, where it is found at ca. 700 cm-l, some 20 cm-l lower, than in the others. (In 3-bromopyridine v12 has become coincident with vq.) A third frequency in this region is the planar ring vibration vgb, found characteristically at around 615 cm-l for 2- and 3-substituents and around 660 cm-l for 4-substituents; it is in all cases especially strong in the Raman spectrum: in the substituted compounds it is only weak in the infra-red, but appears strongly in the spectrum of the other compounds. It appears that the most convenient diagnostic test of the substituent position in monosubstituted pyridines is the use of this band together with the umbrella vibration, vlOb. The lower out-of-plane ring bending mode vn is more X-sensitive in the 2- and 3-substituted compounds than in the 4-substituted compounds owing to the reduced

564

J. H. S. GREEN, W. KYNASTON and H. M. PAISLEY

symmetry. It is of medium intensity for 2-substituted compounds, weak for 3- and strong for 4-substituted compounds. KATRITSKY and his co-workers have studied the “characteristic” bands of many 2- [ 151, 3- [16] and 4- [ 171 substituted pyridines and recorded their position and intensities as measured for 0.2 M solutions in chloroform, (cf. also SHINDO [lS]). In all cases, the columns 2,3,4 and 5 of their tables agree with the assignments to %,, v8b? v19a and ‘19b? respectively, of the present work. For the 4-substituted compounds [17], their column 6, a band at 1067 f 3.5 cm-l is the a,, essentially #I(CH), mode vlSa; their revised assignment of column 7 (993 f 2.5) to be a, ring vibration v1 is confirmed here, whilst their column 8 is confirmed as the b, umbrella mode. In the case of the 2-substituted compounds [15] their columns 6, 7 and 9 are the rows y3, vga and visa of Table 11; column 8 is vlsb, the additional bands listed for the halogen compounds being the X-sensitive vibration vr3. However, the band at 994 & 4 (column 10) is, from its considerable intensity in the Raman spectrum, obviously vr and not a y(CH) frequency. For the 3-substituted compounds [16] column 6 was partially obscured by solvent but is clear from the present work at ca. 1190 cm-r assigned to yga. On the other hand, the band at 1125 f 6 (column 7) is not here taken as a fundamental, though that at 1081 f 10 is so taken and again the additional band in this region for the halogen compounds is yr3. The attribution of column 10 (995 & 5) to the ring vibration is confirmed here and, finally, column 11 is no doubt the umbrella frequency appearing just above 800 cm-l in a few compounds. Note added in prooj-One of us (W. K.) has now measured the depolarlsetion ratios of the Raman hnes of 4-methylpyrrdine, using an improved source unit. The values are included in Table 1 and confirm the assignments made in Table 10. Similarly, the partial data for 3-chloropyrrdine in Table 5 are m agreement wrth the assignments of Table 11. Acknowledgements-We thank Mr. R. R. COLLERSON and Mr. D. HARROP for the purification of 4-chloropyrldme, and Mr. P. W. B. BARNARD for experimental assistance.