Vibrational studies of monosubstituted halogenated pyridines

Journal of Molecular Structure, 42 (1977) @Elsevier

Scientific

VIBRATIONAL PYRIDINES

Publishing

Company,

37-49 Amsterdam

-

Printed in The Netherlands

STUDIES OF MONOSUI3SI’ITUTED HALOGENATED

HUSSEIN

ARDEL-SHAFY,

Chemistry (U S.A.)

Division,

HOWARD

PERLMU’ITER

New Jersey Institute of Technology,

and HOWARD Newark,

KIMMEL**

New Jersey 07102

(Received 25 May 1977)

ABSTRACT Infrared and Raman spectra of 2- and 3Godopyridine and 2-fluoropyridine have been measured. Complete vibrational assignments for the three molecules are proposed. Linear relationships between Xsensitive vibrations and structural parameters are shown to be valid for the 2- and 3-halopyridines as well as for the halobenzenes. The assignments for the halopyridines are correlated with one another and with those for the halobenzenes It is concluded that, in most cases, the ring vibrations of the pyridine derivatives are ~10~ely parallel to those of the phenyl derivatives, but the hydrogen deformation frequencies are generally higher in the substituted pyridines than in the corresponding monosubstituted benzenes.

INTRODUCTION Studies

these frequencies have reported [l-11]. The monosubstituted benzene molecules, some of the simpler aromatic systems have studied extensively. Whiffen’s [1,2] shown that, of the thirty fundamental of vibration ring, twenty-four vibrations are essentially independent of the substituent attached to the ring, other six are sensitive i.e., X-sensitive vibrations. Varsanyi which includes comforce fields.

*Presented in part at the 29th Symposium on Molecular Structure and Spectroscopy, the Ohio State University, Columbus, Ohio, U.S.A., June, 1974, Paper WH9. **To whom correspondence should be addressed.

38

Some correlations have been observed for the X-sensitive vibrations and molecular properties of molecules with the general formula (phenyl),X (n = 1,2,3 or 4). In a study of the q-vibration, Kross and Fassel[4] found that for each value of n, the frequency is linearly related to the square of the electronegativity. The Gordy Rule [12] was used by Kimmel [5] to study the nature of the t-vibration of these molecules. It was found that the general form of the Gordy Rule can be used for a particular value of n and that the t-vibration can be considered a pure (phenyl)-X stretching vibration, except for those molecules formed from the first-row elements. The purpose of this investigation was to extend the study of the vibrational

spectra and structural correlations of related aromatic systems to monosubstituted pyridines. In the benzene ring, all six positions are equivalent so that, for any substituent, there is only one monosubstituted phenyl derivative However, a substituent can take a position on a pyridine ring that is ortho, meta, or para to the nitrogen atom in the ring. Thus for any substituent, three isomers can exist and it should be possible to correlate molecular parameters with vibrational frequencies for the position of the substituent as well as for substituents in the same group of the Periodic Table, e.g., halopyridines. Vibrational assignments have been reported for the methylpyridines [6,8,9] and for chloro- and bromopyridines [ 91. Raman displacements have been measured for 2- and 34luoropyridine [ 131 and assignments based on these measurements have been reported 191. The complete assignments for the methyl-, chloro- and bromopyridines and the partial assignments for 2- and 3-fluoropyridine have been correlated with one another and with the assignments for the deutero compounds [9]. However, the usefulness of correlations involving the methyl group or first row elements is rather limited due to extensive vibrational coupling between the substituent and the ring [ 53. In this study, the infrared and Raman spectra of Z-fluoro-, and 2-, and 34odopyridine have been measured and complete frequency assignments are made. Vibrational assignments for the halopyridines are correlated with one another and with those of the monosubstituted halobenzenes. Relationships between the vibrational frequencies of the halopyridmes and molecular parameters were investigated.

EXPERIMENTAL

The preparation of 2-iodopyridine was patterned after the halide exchange method of Finkelstein [ 14]_ In this procedure, 2-bromopyridine (Aldrich Chemical Co.) was reacted with 50% hydriodic acid. The 3-iodopyridine was prepared by the diazotization of 3-aminopyridine (Aldrich Chemical Co.) [ 15].2-Fluoropyridine was obtained commercially (Aldrich Chemical Co.). The infrared spectra were recorded using an IR Perkin-Elmer Model 467 double bezm spectrophotometer in the range 4000-250 cm-‘. A Jarrel-Ash

39

Model 400 Raman spectrometer, using a Spectra-Physics Argon Ion Laser with 2 watt maximum excitation power, was used to record the Raman spectra in the 4000-100 cm” range. The infrared spectra of the liquids were studied as capillary films using a cesium iodide cell. The solid was studied as a mull and in a potassium iodide disk. Standard liquid and solid sampling methods were used for obtaining the Raman spectra. RESULTS

The observed vibrational tiequencies and their assignments for 2-fluoro-, 2- and 3-iodopyridine are assembled in Tables 1,2 and 3, respectively. The assignments were made by comparing the spectra of the fluoro- and iodopyridines with the available spectra of the 2- and 3-halopyridines 191. frequency assignments for 2-fluoropyridine [S] based on Raunan spectrum measurements 1131 have been reported. The previously reported Raman spectrum of 2fIuoropyridine is also shown in Table 1. Most of the vibrational assignments shown in Tables l-3 are straight-

forward and require no further discussion. The assignments for the X-sensitive vibration and the in-plane CH bending mode which occur in the 1090-1070 cm-’ range of the spectra of the iodopyridines deserve some comment. These assignments illustrate the fact that the location of the substituent in the pyridine ring can be as important as the nature of the substituent, even for the vibrations whose frequencies should be unaffected by the nature of the substituent on the ring. The vibrational spectra of 2- and 3-chloro- and bromopyridines [S] (Tables 5 and 6) show that the in-plane bending mode for the S-halo substituents should be at a higher frequency than the one for the 2-halo substituent. Thus, the absorption bands at 1088 cm-’ for 3-iodopyridine and 1073 cm- 1 for 2-iodopyridine are assigned to this mode. The bands at 1078 cm-’ for 34odopyridine and 1088 cm-’ for 24odopyridine must arise from the X-sensitive vibration. Further evidence for the assignments of this X-sensitive vibration will be presented later. The low-frequency Raman spectra of 2- and 3-iodopyridine are also of interest. The spectrum of 2-iodopyridine shows a strong absorption band at 120 cm-’ (Table 2) while the spectrum of 3-iodopyridine shows two medium intensity bands at 123 cm-’ and 114 cm-’ (Table 3). The 2-iodopyridine isomer is a liquid while 3-iodopyridine is a solid. Absorption bands in this frequency range are not reported for the other halopyridines 19; 131. Although no definite assignments for these bands can be made at this time, interactions between the electron clouds of the large iodine atom and the pyridine ring causing a torsional motion may be a possible explanation.

40 TABLE

1

Vibrational frequencies (cm-‘) infrared

RUllEUl

Liquid

Lit. Cl31

Interpretation This study 0’ U(GH) 0’ u(CH) P'v(CH):U'Y(CH) 1598+ 1434= 3032(A') 2X 995=199O(A') 962+995=1957(Aw) 2X 962=1924(A') 2X 872=1744(A') 962+732=1694 (A’): 388-t- 1302=169O
3097(?.h) 3034 3082(m) 3075(m) 3074 3030(m) 1980(w) 1950(w) 192O(vw) 174O(vw) 1692(w) 1671(m) 1657
a’ v(CC) 962+ 620=1582(A")

1598 (vs) 1598 1582(s) 1580 1579
a' v(CC) 1098+477=1575(A") 937 +620=1557(A") 1302+ 230=1532(A"): 1143+ 388=1531(A')

1098 + 418=1516

1610(m) 1492(m) 1473 (vs) 1466(m)

872+620= a’ u(CC.

1434 (vs) 1412 1390(w)

1437

1376
1376

1354(w) 1342(w) 1327(w) 1302(w) 1291(m)

and assignment for 2-fluoropwidine

1303

1259(m)

1303

(A")

1492 (A”)

CN)

1047 i- 1465(A"); 2X 732=1464(A') o'u
Interpretation

Infrared

R-Sill

Liquid

Lit. Cl31

This study

1247(vs) 1240 (s) 1219
1249

1250

1146

1155

1099 1045

1102 1050

996

997

1022(m) 1008(m) 995 (5) 972(m) 962(w) 947 (w) 937(w) 872(m) 856(m) 840 (s) 828 (vs) 809(m) 787 (vs) 779 (vs) 732 (s) 700(m) 620 (s) 558 (s) 477(m) 418(m) 388(w)

965 935 875 841 828

832 792

771 725 686 622 556

230

630 565 479 420 230

a'X-sensitive 2X 620=124O(A') 828+ 388=1216 (A') 787+ 418=1205(A') 962+ 230=1192(A') 620+ 658=1178(A') 937 + 230=1167(A') u'fl(CH) 2X 558=1116 (A') a'fl(CH) a'fl(CH) 620+ 418=1038(AU) 558+477=1035 (A”) 787 + 230=1017 (A') 620-c 388=1008(AI) a'rmg 558 + 418=976(A") a"-y(CH) 558 + 388=946(A') amy a"-y(CH) 620+ 230= 85O(A") 2X 418-836 (A’) a'x-senntive 418+388=806(A") a"MCH) 2X-388= 776(A-) a- @(CC) 47?+ 230=707(A') o'(r(CCC) o'X-sensitive a"@(CC) a"@(CC) a’ X-sensitive o' X-sensitive

a’ B(CH)

872t 732+ 872+

418=1290(A'); 558=129O(A") 388= 1260(A")

DISCUSSION

It would be reasonable to expect that a close analogy should exist between many of the vibrations of monosubstituted benzene derivatives and the corresponding monosubstituted pyridine derivatives. Kline and Turkevitch [ 161 have shown that the ring vibrations of pyridine are closely parallel to those of benzene and benzene-d,. It has further been found that the hydrogen deformation vibrations are widely different and, where identifiable, they usually shift to lower frequencies going from benzene to pyridine [16,17].Th e closeanalogy between the ring vibrations of pyridine

41 TABLE

2

Vibrational

Infrared

frequencies

Raman

(cm-‘)

and

assignment

Interpretation

Licwid

Infrared

RaIllMl

Interpretation

Liquid

3125(w) 3070(m) 3066

(ml

3055 3046(m) 1842(vw)

165O(vw) 1637
for 2-iodopyridine

1444(m) 1418(w) 134O
2X 1560= 312O(A') a'u(CH) a' u(CH) a' u(CH) a’ u(CH) 960-b 882=1842 (A’)! 1088+758=1846(A"k 1445+ 402=1847 (A") 1416+ 232=1647 (A’); 1042+ 612=1654(A') 882+ 758=1640 (A’) 960+ 680=164O(A") 1338 + 259=1597(A'); 988 + 612=1600
1146(w) 1148(m) 1098 (vs) 1088 (VS) 1073 (vs) 1058(w) 1036 (s) 1042 (s) 978 (s) 988 (s) 96Ob-w) 948(vw) 947(vw) 907(vw) 882(w) 758 (vs) 725(w) 69O(vs)

88O(vw)

680 (s) 668
606(w)

259

(s)

232(w) 165 (s) 120 (s)

a’ P
a’ PWW 882+

165=

1047

(A’,

a’ LKCH)

a’ ring a” -y(CH) au -y(CH)

2X 455= 910(A'): 68OC 232=912(A') au y(CH) a"y(CH) a"y(CC) 455 + 232= 687 (A”) a’ X-sensitive

402+ 259=661
and benzene derivatives has been discussed and is based on the spectra of mostly alkyl derivatives [ 181. However, other work on monosubstituted benzene derivatives has indicated that some vibrations in alkyl derivatives will show extensive vibrational coupling and would not be representative for comparison purposes [ 51. Further, the halobenzenes would serve as a good basis for comparison studies [ 1,2]. In this work investigation and comparisons are made between the vibrations of monosubstituted benzene and pyridine derivatives, as well as between the different monosubstituted pyridine derivatives. In his study of monosubstituted halobenzenes [ 11, Whiffen used a code letter designation for the vibration in accordance with the approximate mode diagrams of Randle and Whiffen [Z] and this has since proved very uesful in comparing vibrations of monosubstituted benzene derivatives. In this study, these designations are adopted for the monosubstituted pyridines by comparing the approximate mode diagrams and approximate vibrational descriptions for the phenyl derivatives [ 1,2 J and the pyridine derivatives [8,9]. These designations should prove useful in comparing monosubstituted pyridines and facilitate the comparison of the vibrations of monosubstituted phenyl derivatives with the corresponding vibrations of monosubstituted pyridines. The designations of Whiffen, the Wilson notation [19] and the approximate descriptions of the vibrations for the monosubstituted pyridines are

42 TABLE

3

Vlhational frequencies(cm-')andassignment Infrared

Raman

KIDiak

MUU

3070(m)

3120(m) 3070(m)

3040(m)

3045(m)

1562(m) 1550(m) 1540(m) 1520(m) 1460(m) 1408 (s) 1370(m) 1337(m) 1318(m) 1260(w) 1231(w)

3010(m) 1958(w) 1921(w) 1897 (w) 1860(w) 1833(w) 1728(w) 1565(m) 1554(w) 1540(w) 1511(w) 1460(m) 141O(vs) 1370(s) 1340(m) 1320(m) 1260(m) 1231(m)

3122(vw)

3042(m)

1138(m) 1132(m) 1116(m) 1088(m) 1078 (s) 1030(m) 1005(vs) 994(m) 953 (w) 942 (w)

915(w) 875(w) 789 (s) 700(x%) 686(m) 615(m) 610(m) 436(vw)

1207(m) 1189(m) 1170(m) 1150(m)

3-iodopyridine

Interpretation

2x 1554=3108

(A’)

u’ u(CH)

3065(w)

1215(w)

for

1565(w) 1553(w)

u'u&H) a'u(CH) (I' u(CH) 1554+ 1460=3014(A') 1565 + 390=1955 (A”) 1005 + 915 =1920 (A”) 1207 + 689=1896 (A’); 1460+ 432=1892 (A”) 940+ 915=1855(A');1078+ 788=1866 (A”) 2x 915=183O(A');1565+ 263=1828 (A’) 940 + 788=1728(A');1030 + 700=1730 (A”) 4’ V(CC) a’ Y(CC)

1320+ 220=1540 (A’) 1078 + 432= 151O(A”) 1458(m) 1408(vw)

u’ v(CC, CN) a’ v(CC,CN)

1189 + 183=1372 915 + 432=1347

(A”);940 + 432=1372 (A’) (A”); 1078+ 263=1341 (A’)

a’ u(CC, CN)

183=1261(A");1005 + 263=1268 (A’) 263=1234(A");1005 + 220=1225 (A’) 2 x 612=1224(A-') a'@(CH) a'p(CH) 788 + 390=1178(A');915 + 263=1178(A") 971+ 183=1154 (A’) 915+ 220=1135 (A”) 700+ 432=1132(A') 940+ 183=1123 (A’);689 + 432=1121 (A”)

1078+ 971+

1187(vw)

1115(m) 1088(m) a’ WH) a’ X-sensitive 1078 (m) lOSO 1035 (vs) a'B(CH) 1030(m) 1005 (vs) 1007(vw) u'ring 612+ 390=1002 (A”) 994(m) a” y(CH) 971(m) 689 + 263 = 952 (A’) 953(m) a” y(CH) 940(m) 325 (VW) a" -y(CH) 915(m) BOO+ 183=883 (A’) 885(w) 788 (s) a” -r(m) a” @(CC) 7OO(vs) 692 (m) fi X-sensitive 689(m) u’ a(CCC) 617(w) 612(w) 432 + 183=615(a'I');390+ 220=610 609(m) a” @(CC) 432(w)

(A”)

43 TABLE

3 (continued) Baman

Infrared

Interpretation

Mull

KIDi& 390 (w) 371 (v-w) 260 (VW)

390 371

(w) (vw) 263 232 220 195 183 123 141

(vs) (w) (vw) (m) (m) (m) (m)

a” #(CC) 2 x 183 = 366 a’ Xsensitive

(A’)

a’ X-sensitive a” X-sensitive

shown in Table 4. While monosubstituted benzene derivatives have 30 normal modes of vibration, monosubstituted pyridines have 27 normal modes of vibration. The vibrations that are “lost” in going from the benzene moiety to the pyridine moiety are a C-H stretching vibration, “zj” (A,), a C-H in-plane deformation, “c” (B,), and a C-H out-of-plane deformation, ‘3” (&).

TABLE

4

and approximate descriptionsof vibrations for monosubstituted pyridines

Notation Symmetry

group

Notation

Approximate description

4-subs.

2-subs. 3subs.

Wilson

Whiffen

A,

A'

2 20a 8a 19a

=, =1 k

u(C-H) u(C-H)

m

v(C-C, v(C-C,

-4,

A”

v(C-3

C-N) C-N)

90 1812 1

a

P(C-H)

b

P(C--H)

P

13 12 6a

4 r

t

ring -(C-C-C) X-sensitive X-sensitive X-sensitive

h

r(C--HI

17L7 104

g

16a

w

Symmetry

Notation Wilson

Whiffen

4subs.

Psubs. 3subs.

20b 7b 8b

2,

B,

A’

B,

A"

;’

19b

n

14 3 18b

0

:

6b 15

s

J f

@C-C)

5 lob 4

I@(C-c)l

l1

y

X-sensitive

166

x

7K--HI

group

U

V

Xsensitive

44

The relationship between the electronegat@it~and the frequency of the q-vibration was appbed to th@ Z- and 3-haldpyridties. The results for the h~op~d~~s and the halobimzenes are shown in Fig, 1. As with thephenyl derivatives [ 41, a straight line was obtained for the halopyridines and the location of the substituent on the pyridine ring influenced the slope of the line, The results shown in Fig. 1 indicate an incorrect assignment by Green et al. [Q] for the q-vibration of 3-fluoropyridine, The straight line obtained for the 3-halopyridines (Fig. 1) indicates that the Raman displacement at 1187 cm-’ [13] should be assigned as the q-vibration instead of the 1247 cm-~ line previously assigned to it 191. The Gordy RL& plots for the t-vibration of 2- and 3-halopyridines are shown in Fig_ 2, The plot .-or the halobenzenes is included for comparison. Bond length and electronegativity data for the halopyridines are the same as those used for the halobenzenes [5]. As in the case of the q-vibration, a straight line is obtained for Z-substituted pyridines, Also it is seen that the frequency of the t-vibration for each fhoro derivative deviates from the linear plots. The structural correlations obbtamed for the halopyridines are analogous to those obtained for the halobenzenes. Further, the nature of the linear I260 1250-

lTSO-

(AI

Z-fidlOpyrrdtneS

(6)

3-ildlc~ynd~nes

(C) Halobrrnrenes

IlW_

0

2

4

6

B

10

12

14

lb

[ElecCrmegatwrty)Z

Fig. 1.Plot of (electrone-ga+tivity)2 vs_ frequency for the q-vibration for 2-and 3-halopyridines, and haiobeazenes-

45

4.0

0

4.2 a 4.0

3.8

(A)

3-Halopyridines

(8)

P-Halopyridlnes

(C)

Halobenzenes

3.6

3.4

3.2

Fig. 2. Plot of Gordy’s Rule for 2- and 3-halopyridines

and halobenzenes.

relationship for the halopyridines is dependent on the location of the substituent on the pyridine ring for these X-sensitive vibrations.

Assignments

and trends

The vibrational assignments for the 2- and 3-halopyridines are summarized in Tables 5 and 6, respectively_ The assignments for the halpoyridines with a common location of the substituent on the ring show that the vibrations considered to be insensitive to the nature of the substituent [I, 21 behave in this manner for-the 2-substituted derivatives and for the 3-substituted derivatives. In-each case (and as with the halobenzenes Cl]), it is usually the fluoro derivative that shows the largest frequency deviation from the other halo derivatives (when such deviations exist). Further, comparisons between Table 5 and the corresponding assignments in Table 6 show that for several vibrations, the location of the substituent on the ring may have a significant effect on the frequency, while the nature of the substituent has little effect on the frequency; e.g., the in-plane or out-of-plane C-H bending vibrations. Such comparisons are useful in evaluating previous assignments of frequency data. Thus, Green et al. 191 had assigned a Raman displacement at 1303 cm-’

46 TABLE

5

Vibrationa!

assignmenti

Symmetry species

Arssignments

4CH) 439

2, 2, k m

&CC) v(CC,

a b

PW~l stCW

P q

fing

r f

for the 2haiopyridines

clv)

X-sensitive X-sensitive X-sensitive

24 v(CH)

4CH) 4CC) u(CC.

u(CC,

CN) CN)

ION= 1 dCH> a(CCC) X-sensitive -ACH) r(CHI @(CC) YACHI rtCH> swq #(CC); X-senstive X-sensitive

Vibrational

frequencies

(cm-‘)

2-Fluoropyridine

2-Chloropyridine

2-Bromopyridine

pyridine

3097 3075 1598 2473 1143 1047 995 1247 828 558 3082 3075 2579 1434 1375 1302 1098 620 388 962 872 418 937 787 732 477 230

3075 3060 1580 1452 1150 1045 994 1x20 727 428 3075 3060 1568 1420 1366 1288 2085 618 310 960 881 410 935 763 724 480 190

3070 3050 1573 1450 1150 1043 989 1105 702 312 3070 3050 1563 1422 1350 1282 1079 615 260 960 880 405 932 760 725 470 178

3066 3046 1570 1445 1148 1042 988 1088 680 259 3070 3055 1560 1415 1338 1278 1073 612 232 960 882 402 947 758 725 455 165

2-Iodo

for 2-fluoropyridine as a C-H in-plane bending mode (e-vibration). The Raman line at 1308 cm” in 3-fluoropyridine was assigned to this vibration. If the frequency of this vibration is compared for the other halopyridines, it is seen that the frequency in 2-halopyridine is about 60-70 cm” greater than the frequency of the vibration for the 3.halopyridine with the same substituent. Thus it is more likeIy that the Raman line at 1247 cm-l 1131 should be assigned as the e-vibration in 3-fluoropyridine. Table 7 gives comparisons of frequency ranges for each vibration for 2-, 3- and 4-halopyridines and halobenzenes. The data in Table 7 extends the suggestion of Kline and Turkwitch [16] that the ring vibrations of pyridine are closely parallel to those of benzene. This includes the y-vibration which is considered to be an X-sensitive mode by Whiffen Cl] but a ring mode by Green et al. [S]. The ring mode listed as the u-vibration might be considered an exception since the 2- and 4-substituted pyridines have a frequency

47 TABLE

6

Vibrational

assignments

Symmetry

Assignments

for the 3-halopyridines

species

ii

(a,)

2, 2,

Y(CH) 4CH)

k

u(CC)

m u(CC,

CN)

a P(CH) b P(CH) p ring

lb,)

q r

X-sensitive

=s I II o

u(CH) u(CC) u(CC,CN)

X-sensitive t X-sensitive =q p(CH)

p(CH) d atcH) s a(CCC) au (a,)

(b,)

--

3-Fiuoro-

3-Chloro-

3-Bromo-

3-Iodo-

pyridine

pyridine

pyridine

pyridine

3069 3058 1594 1480 1187 1038 1023 1187a

3079 3050 1573 1463 1190 1040 1015 1093 728 428 3079 3050 1569 1415 1320 1225 1107 612 294 980 915 404 943 795 698 460 199

3075 3052 1574 1463 1190 1025 1008 1087 705 319 3075 3052 1557

3065 3042 1565 1460 1189 1030 1005 1078 689 263 3070 3045 1554 1410 1320 1207 1088 612 220 971 915 390 940 788 700 432 183

818 535 3069 3058

1584 1425

rr(CC, CN)

e

u

Vibrational frequencies (cm-')

1247a 1095

616

h g w i

X-sensitive r(CH) -r(CH) @(CC) r(CH)

;

;::c":

702

y x

@(CC); X-sensitive X-sensitive

244

=Vibrations reassigned in this study

982

410

using frequencies

reported

1417

1320 1220 1095 614 246 980 915 400 945 792 699 447 182 from

Raman

spectrum

[IS].

range about 30-40 cm- 1 higher than the halobenzenes (and about 20-30 cm-’ higher than the S-substituted pyridines). The other comparison between the pyridines and phenyl derivatives reported previously involved the hydrogen deformation frequencies. It has been suggested that these vibrations should shift to lower frequencies going from benzene to pyridine [ 16,171. However, the results of this study show that, in most cases, the hydrogen deformation frequencies are actually higher in the monosubstituted pyridine than in the corresponding monosubstituted benzene (see Tables 5 and 6). The notable exception to this conclusion involves the out-of-plane bending mode labelled the j-vibration, where the frequencies for the pyridine derivatives are significantly lower than those of the phenylderivatives, m the case of the C---N bending modes, the a-vibration for the 2-substituted pyridines and the e-vibration for the 3-substituted pyridines show frequencies somewhat lower than the corresponding vibrations in the monosubstituted benzenes.

48 TABLE

7

Comparisons

of halopyridines

Symmetry species

Axsignments

0’ (a,)

2, 2,

4CW

k

u(CC) u(CC, CN)

m

and halobenxenes

4CW

u NCH) b INCH)

(4)

a” (4

(b,)

ring

P q r t

X-sensitive X-sensitive Xsensitive

2. a, ! n o

WH) I u(CC) v(CC, CN) u(CC, CN)

e d s u h

WH) WH) a(CCC) X-sensitive

g

r(CH) @(CC) r(CH) T(CH) CWC) @(CC); X-sensitive X-sensitive

al i f LJ y x

YW-U

Vibrational

frequencies

(cm-‘)

2-Halopyridines

3-Halopyridines

4-Halopyridinesb

Halobenze

309746 3075-46 1598-70 1473-45 1150-43 1047-2 995-88 1247-1088 828-680 558-259 3082-70 3075-55 157Q-60 1434-15 1375-38 1302-78 1098-73 620-l 2 388-232 962-60 882-72 418-02 947-32 787-58 732-24 480-55 23Q-165

3079-65 3058-42 1594-65 1480-60 1190-87 1040-25 1023-05 1187-1078 818-689 535-263 3079-69 3058-45 1584-54 1425-10 1320= 1247-07 1107-1088 616-2 294-220a 982-71 915a 410-390 945-O= 795-88= 7 02-698 460-32a 244-182

3076-2 3048-35

3072-65 3053-49 1603-157 1499-69 1178+6 1026-15 1010-998 1220-106 807-654 520-260 3100-306 3072-48 1597-71 1460-36 1326-18 1290-55 1068-5 616-3 407-220 965+5 838-29 409-398 990-80 752-30 687-l 501-448 242-166

1575-67 1484-2 1219-6 1064-2 996-2 1103-1091 712-680 414-317 3076-2 3048-35 1566-4 1407-3 1359-39 1316 1080-76 663-2 300-25 6 961-55 914 390 859-36 81-l--05 722 491-82 182

=The Frequency for 3-fluoropyridine is missing. Frequencies for fluoropyridines are usually higher than others in the series, which usually exhibit a broader range of frequencies. bOnly the vibrational spectra of 4-chloro- and 4-bromopyridine are available and included in this column. REFERENCES 1 D. H. Whiffen, J. Chem. Sot., (1956) 1350. 2 R. R. Randle and D. H. Whiffen, Molecular Spectroscopy, Institute of Petroleum, London, 1955, p. 111. 3 B. Varsanyi, Vibrational Spectra of Benzene Derivatives, Academic Press, New York, 1969. 4 R. D. Krosa and V_ A. Fassel, J. Am. Chem. Sot., 77 (1955) 5858. 5 H. S. Kimmel, J. Mol. Struct., 12 (1972) 373. 6 D. A. Long, F. S. Murfin, J. L. Hales and W. Kynaston, Trans Faraday Sot., 53 (1957) 1171. 7 G. L. Cook and F. M. Church, J. Phys. Chem., 61 (1957) 458.

49 8 D. A. Long and W. 0. George, Spectrochim. Act-a, 19 (1963) 1777. 9 J. H. S. Green, W. Kynaston and H. M. Paisley. Spectrochim. Acts. 19 (1963) 549. 10 D. B. CunliffeJones, Spectrochim. Acta, 21(1965) 747. 11 V. I. Brezin and M. D. Elkin, Opt. Spectrosc., 32 (1972) 554. 12 W. Gordy, J. Chem. Phys., 14 (1946) 305. 13 H. P. Stevenson and F. L. Voelz. J. Chem. Phys., 22 (1964) 1945. 14 H. Finkelstein, Ber., 43 (1910) 1528; H. Finkelstein, Cbem. Abstr., 4 (1910) 2441. 15 A. Binz and C. Roth, Ann., 486 (1931) 95; A. Binz and C. Roth, Chem. Abstt., 25 (1931) 3344. 16 C. H. Kline, Jr., and J. Turkwitch, J. Chem. Phys., 12 (1944) 300. 17 L. Corrsin, B. J. Fox and R. C. Lord, J. Chem. Phys., 21 (1953) 1170. 18 L. J. Bellamy, The Infrared Spectra of Complex Molecules, 2nd edn., John Wiley, New York, 1958, Chap. 16. 19 E. B. Wilson, Phys. Rev,, 45 (1934) 706.