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The vibrational spectra of benzene derivatives—I
Spectrochimica Acta, 1981,Vol. 17,pp. 486 to 602. Pergamon PressLtd. Printedin NorthernIreland
The vibrational spectra of benzene derivatives-I Nitrobenzene, the benzoste ion, alkali metal benzoates and salicylates J. H. S. GREEN,
W. KYNASTON and A. S. LINDSEY National Chemical Laboratory, Teddington, Middlesex (Received 8 January 1961)
Abstract-Infra-red and Raman spectra have been recorded for nitrobenzene and a complete vibrational assignment is given. The infra-red spectra of the alkali metal benzoates have been recorded and, together with the Raman spectra data, a substantially complete assignment is given for the benzoate ion. The infra-red spectra of the alkali metal salicylates, salicylic acid and o-nitrophenol have been measured, correlated and partial assignments made. Examination of the difference Av between the symmetric (v.) and antisymmetric (v,,)COz stretching frequencies in the two series of salts shows that, whereas for the benzoates Av increases steadily from lithium to caesium, for the salicylates, with the exception of the lithium salt, it is either constant or decreases slightly; v,, is 20-30 cm-l higher in the salicylates than in the corresponding benzoates; v, is lower in lithium salicylate than in lithium benzoate, but in rubidium and caesium salicylate V, is higher than in the corresponding benzoate. No simple linear relationship is found between Av and the polarizability of the free ions or the electronegativities of the alkali metals.
Introduction the infra-red spectra of organic carboxylic acids have been examined in some detail [l-3] the spectra of the metal salts of such acids have received much less attention. The effect of the metal ion on the spectra is of interest and recently attempts have been made to correlate band shifts with various properties such as ionic deformalities, ionic radii and electronegativity values [4-61. The infra-red spectra of the alkali metal salts of benzoic and salicylic acids, available from earlier work [7] have therefore been studied in more detail to investigate the frequency changes arising from changes of the alkali metal. To attempt a complete assignment for the benzoate ion, the Raman spectra of aqueous solutions of lithium and sodium benzoate have also been recorded. In view of the close similarity between the assignments for the isoelectronic pairs CH,NO,, CH,CO,and CF,NO,, CF,CO,-, a study of the vibrational spectrum of nitrobenzene was made and used to clarify the assignments for the alkali metal benzoates. Similarly, a comparison of the spectra of o-nitrophenol and the alkali metal salicylates was helpful in making the assignments for these compounds. ALTHOUGH
Experimental (a) Materials Nitrobenzene
was purified by vacuum
distillation
and examined
immediately
[I] D. HA&I and N. SHEPPARD, PTOC.Roy. Sm. (London) A 216,247 (1953); S. BRATO&D. HADZI and N. SHEPPARD,Spectrochina.Acta 8, 249 (1956). [2] M. ST. C. FLETT, J. Chem. Sot. 962 (1951). [S] F. GONZ~~LEZ-SANCHEZ, Spectrochim.Acta 12, 17 (1958). [4] R. THEIMERand 0. THEIMER,Monatsh. Chem. 81, 313 (1950). [5] R. E. KAQARISE,J. Phys. Chem. 59, 271 (1955). [S] B. ELLIS and H. PYSZORA,Nature 181,181 (1958). 171 J. L. HALES, J. I. JONESand A. S. LINDSEY, J. Chem. Sot. 3145 (1954). 486
The vibrational spectra of benzene derivatives-I
after a further distillation. o-Nitrophenol was recrystallized from alcohol-water and dried, and had m.p. 46~0°C. The lithium, sodium and potassium benzoates, and sodium and potassium salicylates were reagent grade materials (B.D.H. Ltd.). Rubidium and caesium benzoates were prepared by neutralization of AR benzoic acid with standard lithium salicylate by neutralization of rubidium or caesium hydroxide solution; salicylic acid with standard lithium hydroxide; and rubidium and caesium salicylates by adding the carbonate solution to a known quantity of salicylic acid dissolved in methanol; in all cases followed by evaporation to dryness. All the salts were dried at 110°C. Analysis for the metal gave the following. Benzoates, found: Li, 5.4; Na, 16.0; K, 24.4; Rb, 41.3; Cs, 51.5; talc. for Li, 5.4; Na, 16-O; K, 24.4; R’b, 41.4; Cs, 52.3%. Salicylates, found: Li, 4.9; Na, 14.4; K, 22-l; Rb, 38.5; Cs, 48.9; talc. for Li, 4.9; Na, 14.4; K, 22.2; Rb, 38.4; Cs, 49.2%. (b) Spectra The Raman spectra of nitrobenzene and of saturated aqueous solutions of lithium and sodium benzoate were obtained using a Hilger FL1 Raman source and E612 spectrograph, and were recorded photoelectrically and photographically to confirm certain weak bands. Infra-red spectra of nitrobenzene, and of lithium and sodium benzoates as mulls in Nujol, were taken on a Grubb-Parsons GS 3 instrument. The other compounds were examined as mulls in Nujol and in hexachlorbutadiene and, in some cases, as disks in potassium chloride using a double-beam grating spectrometer [8].
Results and discussion Nitrobenzene Nitrobenzene is not planar [9] but the departure from C,,-symmetry will not be considerable. As with aniline [lo], many of the phenyl ring modes behave according to the rules of this point group for which the thirty-six vibrations would comprise 13a, + 4a, + 12b, + 7b,. The modes additional to the normal thirty for a monosubstituted benzene are the NO, symmetrical stretching and deformation (a-J, the torsion about the C-N bond (a,), the NO, antisymmetrical stretching and in-plane rocking of the NO, group (b,) and the out-of-plane rocking (b,). For C,-symmetry with the plane of symmetry perpendicular to the phenyl ring, the a,- and b,-classes combine to give 20a’ vibrations and the a2- and b,-classes similarly give 16a” vibrations. Partial assignments have been made by earlier workers [ll, 121. The vibrational frequencies are given in Table 1 where the infra-red spectrum of the vapour [ 131 is included, since in some cases the band contours provide valuable additional information, but the region 2000-3000 cm-l is omitted since several possibilities exist to explain the combination bands found there. The present work confirms the presence of a weak Raman line [ll] at 252 cm-l and a very weak line [14] at ca. 420 cm-l, corresponding to the weak band at 420 cm-l in the infra-red [Sl [9] lo] 111 121 131 141
J. L. HALES, J. Sci. In&. 36, 264 (1959). K. E. REINERT, 2. Naturforsch. 15~1, 85 (1960). J. C. EVANS, Spectrochim. Acta IS?428 (1960). H. WITTEK, 2. physik. Chem. (Lezpig) B 52, 315 (1942). LandoltBGrnstein Zahlenwerte und Funktionen Bd. I, Teil 2. Springer, Berlin (1951). R. MECEE, Documentation of Molecular Spectroscopy No. 3368. Butterworths, London (1958). J. BEHRINGER, 2. Elektrochem. 62, 544 (1958).
487
J. H. S. GREEN, W. KYNASTON and A. S. LINDSEY Table 1. Vibrational Raman Liquid (cm-l) *
Liquid
spectrum of nitrobenzene
-
Infra-red Vapour [ 121
Assignment t
-_ 3081(m)
(0.46) (2000-3000
c
3096(m) 3068(m) -l region omitted) 1961(w) 1923(w) 1908(w) 1876(w) 1842(w) 1808(w) 1794(w) 1770(w) 1742(w) 1733(w) 1707(w) 1675(w) 1616(w)
3086( 1)
1597(s) 1586 1575(w)
0.53
1603(s) 1585(s)
1610(3)
1523(m)
0.92
1527(vs)
1550( 10)
1476(m)
0.62
1475(s) 1453(sh)
1486(3)
1412(m) 1380(w) 1358(w) 1345(vs)
dP
1412(m) 1379(sh)
dP (?) 0.19
1351(vs)
1357(10)
1316(s) 1307(s)
1309(2)
al and b, fundamentals
977 + 977 = 1954(A,) 990 + 935 = 1925(A,) 934 + 977 = 1911(B,) 934 + 934 = 1868(A,) 837 + 990 = 1827(B,) 837 + 977 = 1814(A,) 794 + 990 = 1784(A,) 794 + 977 = 1771(B,) 1069 + 677 = 1746(A,) 794 + 935 = 1729(A,) 1002 + 704 = 1706(A,) 837 + 837 = 1674(A,) 1002 + 612 = 1614(B,); 935 + 677 = 1612(B,) a, fundamental b, fundamental 793 + 793 = 1586(A,); 852 + 710 = 1562(A,) b, NO, antisymmetric stretching fundamental a1 fundamental 612 + 852 = 1464(B,); 612 + 837 = 1449(A,) b, fundamental 532 + 852 = 1384(B,) 678 + 678 = 1356(A,) al NO, symmetric stretching fundamental b, fundamental b, fundamental 610 + 678 = 1288(B,) 252 + 1000 = 1252(B,) 611 + 611 = 1222(A,)
1283(w) 1247(m) 122O(vw)
1216(w) 1172(s)
P
1174(s)
1161(s)
P
1161(s)
1108(s)
0.25
1107(s)
1074(m)
P
1244( 1) 1183(2) 1176(2) A 1171(2) 1 116O(vw sh)
a1 fundamental b, fundamental n1 fundamental 397 + 704 = llOl(B,) 397 + 678 = 1075(A,)
1094(s)
-
(w) = weak; (m) = medium; (8) = strong; (sh) = shoulder; (v) = very. * Quantitative depolarization data from [ 111. t Species designations as for C,, symmetry, cf. Table 2.
488
--
The vibrational spectra of benzene derivatives-I Table 1-(co&.)
Raman (cm-l) * Liquid
Liquid
Infra-red Vapour [ 121
1069(s)
1053(w)
1021(s)
P
1004(s) 988(w)
0.11
852(s)
0.23
793(m) 710(w) 678(w) 610(s) 532(m) 435(w) 397(m) 252(w) 176(m)
0.72
1020(s) 1002(m) N99O(VVW) 977(w) 935(s) 852(s) 837(vw ah) 794(s) 704(s) 677(s) 612(w) 532(m) 420(w)
dP (‘I) 0.87
1065( 1) 1069( 1) B( ?) 1074( 1) 1 1030(2) 1025(2) A 1018(2) 1
933( 1) C(P) 857(5) 852(5) A 847(5) I 792(4) c 701(8) C 682(sh) 591(l) 519(O)
P dP
Assignmentt
5, fundamental
al fundamental a1 b2 as b,
fundamental fundamental fundamental fundamental
a, fundamental
b 2 fundamental b, fundamental b, fundamental a1 fundamental b, fundamental b, fundamental b, and b, fundamentals a1 and as fundamentals b, fundamental b, fundamental
-
spectrum of the liquid. WITTEK [ 1 l] reported two Raman lines at 512 and 535 cm-l but other work and the present results reveal only one line corresponding to the infra-red band in the liquid at 532 cm-l, the value for the vapour being 519 cm-l. The band is definitely unsymmetrical, however, as is that recorded by MECKE [ 15]; his reported bands at ca. 410 and 450 cm-l presumably correspond to the values 397 and 420 cm-l of Table 1. Other tabulated values are in good agreement with previous measurements. Assignments of all the observed frequencies are made in Table 1 on a basis of C,, symmetry and the fundamentals are collected in Table 2, where the approximate description of the modes is that of WHIPBEN [16] and the numbering of the corresponding benzene frequencies is that of WILSON [ 171. Only the usual rather arbitrary assignment of the CH stretching frequencies is possible. The a,-component of the pair of CC stretching frequencies derived from Q, in benzene is readily assigned at 1475 cm-l and the b, component might be assigned as the weak shoulder at 1453 cm-l, but the band at 1412 cm-l with a corresponding depolarized Raman line of the same value seems more likely. This frequency is therefore somewhat lower than in other monosubstituted benzenes. The remaining “characteristic” frequencies in the $- and &-classes [ 181 are readily [15] [16] [17] [ 181
R. MECKE, Documentation of Molecular Spect~08c~pyNo. 2409. Buttmworths, London (1957). D. H. WHIFFEN, J. Chem. Sot. 1350 (1956). E. B. WILSON, Phys. Rev. 45, 706 (1934). R. R. RANDLE and D. H. WHIFFEN, Molecular Spectroscopy p. 111. Institute of Petroleum, London (1955).
489
J. H. S. GREEN, W. KYNASTON and A. S. LINDSEY Table 2. Assignment
of fundamentals
for nitrobenzene and the benzoate ion
Mode [16] and No. [17]
a’$
al
ax
20a 2 13 8a 19a
/VW
18a 12 7a 6a 1
NO,, NO,,
NO,,
Y(CH) IWH) ZJ(CH) WV X-sens. X-sens. CO, rock
b1
NO,, NO,, a2
v(CH) v(CH) +(CH) v(CC) v(CC) BWH) ring X-sens. X-sens. X-sens. CO,- sym. stretch CO, sym. deform
5s
_
9a
20b 7b 8b 19b 14 3 ;:EI; 9b 15 /WW 6b a(CCC) X-sens. 18b CO, antisym. stretch CO, rock 17a 10a 16a
Benzoate iont
3096 3081 3081 1603 1475 1174 1020 1002 1107 852 397 1351 677
3088 3073 3073 1601-1609 1483-1513 1172-1181 1015-1026 998-1006 1100-1107 828-846 404 1380-1427 673-678
990 935 794 704 176 420 532
-990 919-940 812-819 705-720 170 418-422 526-543
5 17b lob 4 11 16b
WH) v(CH) 4CC) 4CC) v(CC)
Y(CH) Y(CH) WC)
Nitrobenzene*
3081 3068 1585 1412 1316 1307 1161 1069 612 252 1527 420 or 397$ 977 837 397 5
3073 3036 1593-1597 1407-1427 1305-1314 1270-1282 1158-1163 10641073 617 1552-1561 404 5 973-984 815-840 404 $
* Liquid state values. 7 Ranges of observed values, Table 3. # Species designations assuming Ca,-symmetry, with plane of symmetry perpendicular to that of phenyl ring. 8 Frequency used again.
assigned. All the X-sensitive vibrations in the a,-class involve some CN stretching; they are assigned on a basis of the polarization and band contour data and are (That at 852 cm-l has previously consistent with the values for related compounds. [19] been identified as the CN stretching mode.) The a2 out-of-plane ring deformation v16, is almost certainly around 400 cm-l and in nitrobenzene lies at the same [I91 R. R. RANDLE and D. H. WHIFFEN, J. Chem. Sot. 4153 (1952).
490
The vibratioml spectra of benzene derivatives-I
frequency as the lowest +-mode. A detailed analysis of the combination bands in the region 1675-1960 cm-r in terms of the out-of-plane CH deformations y(CH) modes is given in Table 1 and confirms [20] the assignments made for these frequencies, all of which seem to be observed in the present spectrum of the liquid. Of the frequencies arising from the nitro group, the symmetric and antisymThe symmetric deformation fremetric stretching modes are readily assigned. quency is expected in the range 650-700 cm-l and is assigned here to the strong infra-red absorption at 677 cm-l, in agreement with BEHRINGER [14]. The corresponding Raman line is weak and recorded as depolarized [ 1 l] but with a query and we feel justified in ignoring this. That the strong infra-red band at 704 cm-l is the b, out-of-plane ring deformation Y* is shown by the C-type contour appearing in the vapour spectrum, with the 682 cm-l band present on one side of it. The out-ofplane rocking mode of the NO, group is expected to have a higher frequency than the in-plane mode because, (a) the moment of inertia of the group about an axis perpendicular to its plane is much greater than that about an axis in its plane [21], and (b) the repulsion in the potential energy function for the out-ofplane mode is probably greater than that for the in-plane mode [22]. Therefore the band at 532 cm-l is taken as the out-of-plane rocking mode; the in-plane mode is evidently at the same frequency as some other mode, probably at 420 or 397 cm-l. (In trifluoronitromethane, however, the two modes are apparently both at 400 cm-1 [23].) No evidence is available concerning the a,-torsional mode.
The infra-red spectra of the solid alkali metal benzoates (Fig. 1) are summarized and correlated in Table 3, together with the Raman frequencies observed for saturated aqueous solutions of sodium and lithium benzoates. These latter values are in reasonably good agreement with those reported by FXNKEL’SHTEIN and SHORYGIN 1241, but in the present work neither the lines at 463 and 633 cm-’ reported by them with a query, nor their weak line at 497 cm-r were observed. They reported a line at 1142 cm-l compared with the present value of 1134 cm-r: this line is anomalous and does not appear in the infra-red spectra of the salts and is absent in nitrobenzene. The infra-red spectrum of sodium benzoate is in satisfactory agreement with earlier work [25-271 but the present results show more detail. All the spectra show three aromatic Y(CH) bands unchanged with variation of the metal ion. Indeed, the expected parallelism between nitrobenzene and the benzoate ion is very close, especially in the region 800-1300 cm-l, and extends to some of the combination bands. In spite of the absence of band contour and depolarization data, an assignment, complete apart from one frequency, can be __._.. [20] D. H. WHIFFEN, Spectrochim.Acta 7, 263 (1955).
[21] A. J. WELLS and E. B. WILSON, J. C’hem. Phys. 9, 314 (1941). [22] K. ITO and H. J. BERNSTEIN, Can. J. Chem. 84, 1’70(1956). [23] J. MASON and J. DUNDERDALE, J. Chew&.Sot. 759 (1956). [24] A. I. FIN~EL’SHTEIN and P. P. SHORYOIN, D0kkz.d~ A&d. Ncmk S.S.S.R. 73, 759 (1950). 1251C. DWAL, J. LECOMTEand F. DOUVIL~, Arm phys. 17,5 (1942). [26] R. MEG=, ~0~~~~~~~ ofMolmulw S~~t~~~#~ No. 4823. Butterworthe, London (1959). [27] M. DAVIES md R. L. JOXES, J. Chem. Sm. 120 (1954). 2
491
-I**‘<*
1500
Fig. 1. Infra+A
f
’
moo qoo cm
BOO
xx)
spootra of &caXi metal benzoatay: (a) lithium, (b) LW.%ZZ~~ (c) potassium, (d) rubidru, (0) OSMS~UW
492
The vibrational spectra of benzene derivatives-I
Table 3. Vibrational snectra and as&nment of alkali met& benzoates Infra-red
Raman*
Av
Li
(cm-l) 3074
1544(m)
1392(s)
1300(w)
1155(m) 1134(m) 1023(m) 1001(s) 843(m)
617(m) 404(w)
3090(w) 307 1(w)
CS
3088(w)
1903(w) 1805(w) 1773(w) 1621(w)
191 l(w) 1800(w) 1775(w) 1621(w)
1618(w)
1615(w)
1602(s) 1597(sh) 1561(s) 1523(w)
1595(s) 1 1552(s) 1520(sh)
1594(s) 1 1552(s) 1624(sh)
1609(sh) 1595(s) 1555(s) 1527(m)
160l(sh) 1593(s) 1553(s) 1527(m)
1489(w) -
1488(w)
1502(w)
1513(w) 1440(w)
1483(w) 1447(w)
ilEl”{ 3036(z)
1427(s) 1406(s)
1413(s)
1314(w) 1304(w) 1282(w) 1237(w) 1181(w) -
1305(w) 13Ol(sh) 1270(w) 1236(w) 1180(w) Nl150(w) 1101(w) 1065(m) 1026(m) 1006(w) 974(w) 919(w) 845(m) 840(m) 819(m)
1107(w) %[:j lE~~1 940(w) 846(m) 828(m) 727(s) 720(s) 692(m) 678(m) 543(s) 422(m)
709(s)
683(m) 680(m) 617(w) 526(m) 418(m)
I
:%;:I 1926(w) 1797(w)
1407(s) 1395(s) 1384(s) I
14ll(sh) 1386(s) 1347(w)
1380(s) -
1309(w)
1311(w)
1270(w) 1231(m) 1174(w) .~1161(w) 1100(w) 1065(m)
1280(w) 1222(w) 1176(w) 1156(w) 1105(w) 1065(m) 1016(m) 1000(w) 973(w) 830(m) 812(w) 73l(sh) 719(s) 715(s)
819(m) 713(s) 705(s) 686(m)
- .. a, and b, fundamentals 2v17&41) $‘!3+ %&‘U %?O + %,b(&) r1o.xi- vl,a(Alf Blob+ v,W,f Vie -!- v&B& v172t -I- CO, sym. def. (B,} at fuudamental, rsa b, fundamental, rgb antisym. CO, str. (6,) vba + CO, sym. def. (A,) a, fundamental, visa
1791(w)
-
:E;m’ 970(Z) 920(w) 836(m)
Assignment f
3096(w) 3073(w) 3043(w) I 1979(w) 1927(w)
3088(w) 3073(w) 3036(w) 1954(w)
1973(w) 1927(w) 1823(w) 1793(w) 1619(m) 1600(s)
Rb
K
Nrt
v6b
-t
vaa(B,);
V@ + vloa(-4~) b, fundamental, vlOb sym. CO, str. (ai) 2 x CO, sym. def. (-4,) b, fundamental, ri4 b, fundamental, vQ
1280(w) 1238(w) ~1172(W) 1105(w) 1064(m) 1015(m) 998(w) -
2rse(A,f a, fundamentrtl, vpo 6, fundamental, vpb see text ox fundamental, v,* b, fundamental, vxs a, fundamental, visa a, fundamental, vl$ a, fundamental, vlTo b, fundamental, vlra ai fundamental, vBo oB fundamental, vlaa b, fundamental, vloB
828(m) 815(w) 731(w)
b, fundamental, v4 a, f~d~ment&l
678(m) I
b, fundamental, vgb 6, fundamental b, fundamental, vlab (t2 and b, fundamen-
tals, vlao
170(m)
-
b, fundamental, vll
* Obteined from saturated aqueous solutions of sodium and lithium benzortte. t Notation as for nitrobensene, cf. Table 2.
493
J. H. S. GREEN, W. KYNAS~ON and A. S. LINDSEY
made of the fundamentals and is correlated with that of nitrobenzene in Table 2, where the range of observed frequencies is shown for each fundamental. No explanation for the 1134 cm-l Raman line can be found in terms of observed Raman frequencies, but the following combinations seem possible: err + v~,~ (B,); viGb+ y4 (A,); CO,- rock (i-p) + yBb(A,). The analysis of the bands in the region 16152000 cm-l again provides support for the y(CH) assignments and indicates that an unobserved b,-frequency (vJ is at ca. 990 cm-l. The lowest b, I/(CH) frequency (v& is apparently some 20 cm-r higher in the benzoate ion than in nitrobenzene. Whilst the antisymmetric CO, stretching frequency remains unchanged at 1555 cm-i throughout the series lithium to caesium, the symmetric frequency Table 4. Vibrational
freauencies for nitro and cerboxvlate
NO,, CO,
NO,, CO, Sym.
1gym. def.
str.
C-N,
e~oups in some compounds
NO,, CO,
c-o
Antisym.
Stretch
NO,, CO,
str.
i-p rock
NO,, CO, o-p rock
I-
CH,NO, [21] CH,CO,[22] CF,NO, [23] CF,CO,[28] C,H,NO, C,H,CO,o-HOC,H,NO, o-HOC,H,CO,01
657 645 604 601 677 680 665( ?) 670(?)
1377 1425 1315 1435 1351 1392 1315 (a) .
1562 1578 1620 1681 1583 1560 1536 1582-1587
918 924 863 844 852 828 820 809-816
i
-
481 465 400 410 397(or 420) 4OO(or 420) -
608 615 400 437 532 52&543 -
-
shows a steady decrease, crossing and merging with the b, v(CC) mode (v& in the sodium salt; it is apparently split in the potassium salt. [This b, Y(CC) frequency is the higher possibility exists in two of the salts of 1440 assigned as in nitrobenzene; and 1447 cm-l but these frequencies are attributable to the same combinations as in nitrobenzene.] A smaller trend is observed in one of the v(C-0) modes, vrsa, which decreases steadily from 846 to 828 cm-l, together with a less certain trend in the higher a, p(CH) frequency (vea) from 1023 to 1015 cm.-i Some extra frequencies arise in the region 670-730 cm-l in a manner varying eratically between the salts. The symmetric CO, deformation frequency seems to be split in three of the salts and, except in sodium benzoate, an extra band appears just above the b, out-ofplane ring deformation v4. These are probably related to the physical state of the material, and the extra band may be a combination of a lattice mode with v4. The vibrational frequencies associated with the nitro and carboxylate groups in nitrobenzene and the benzoate ion are collected in Table 4, together with those for other similar isoelectronic pairs. Group frequency shifts in the vibrational spectra of solids may be due to a number of factors which at present can be assessed only on a qualitative basis. In the present case they may be summarized as follows: (a) Effects arising from variation of the spatial configuration of individual molecules, and ionic packing in the crystal. Ionic packing will be related normally to the cation/anion radius ratio which gives an approximate measure of the degree of shielding of the cations by the anions. [28] R. E. ROBINSON and R. C. TAYLOR, S~ectrochim. Acta Dr. R. C. TAYLOR.
494
15,
764 (1959);
private communication
by
The vibrational spectra of benzene derivatives-I
(b) Effects arising from electronic shifts due to intra- and inter-molecular Electronic shifts may arise through the polarization and crystal field interactions. inductive and electromeric effect of substituents, through electronic deformation of the anion by the cation (which can be related to the electronegativity value of the metal), through intermolecular field effects (which may be partly dependent on the degree of shielding of the cation) and through chelation effects where suitable substituents make it possible. (c) Effects arising from variations of the mass of the cation. Factors affecting the spectra of alkali metal benzoates are therefore likely to arise from variations of Table 5. Symmetric and antisymmetric
CO, stretching frequencies of the alkali metal benzoates and salicylates
1 Benzoates
Salicylates Antisym.
str.
Av
ionic packing, electrophilic nature of the group -CO,M+, electronic deformation of the benzoate anion, and mass of the cation. The assignments made above for the symmetric and antisymmetric CO, stretching frequencies show that the difference (Av) between these frequencies steadily increases through the series lithium to caesium (Table 5). STIMSON [29] has previously pointed out that this difference is always greater for the potassium than for the sodium salts of benzoic and aminobenzoic acids, and the present results extend this observation. It was also suggested that this separation is a measure of the deformability [30] of the anion by the cation. However, using the values of Table 5, plots were made of: (a) Av against the polarizability values of the free ions [31]; (b) Av against the electronegativity values of the alkali metals [32]; and (c) the symmetric frequency against the electronegativity values. None of these revealed any simple linear relationship. o-Nitrophenol,
salicylic acid, alkali metal salicylates
The infra-red spectra of o-nitrophenol, salicylic acid and the alkali metal salicylates shown in Fig. 2, and the vibrational frequencies are summarized in Table 6> where the numbered rows indicate the correlations made between the various compounds. For o-nitrophenol, the Raman frequencies, although obtained [29] M. H. STIMSON, J. Chem. Phys. 22, 1942 (1954). [30] K. FAJANS, Radioelements and Isotopes: Chemical lbvea Hill, New York (1931). [31] K. FAJANS and G. Joos, 2. Physik 23, 1 (1924). [32] W. C~IRDY, J. Chem. Phya. 14, 305 (1946).
496
and Optical Properties
Ch. IV.
McGraw-
J. H. S. GREEN, W. KYNASTON and S. A. LINDSEY
30002500 2ooo
1500
loco 900
800
7ccl
cm-’
.Fig. 2. Infrs-red spectra in the solid state of (8) o-nitrophenol, (b) ealicylic acid, and the mlicylates of (c) lithium, (d) sodium, (e) potassium, (f) rubidium, (g) caasium.
496
[33f E.
6
Chwa.
74, 271 (1943).
1466(s)
HERZ
and
1421(W)
1465(s)
1644(m) 1593(s)
1780(w)
>
1awv)
1856(W)
1940(w)
>
1468(s)
1488(s)
1464(s)
1486(s)
1507(ah)
1%33(m) 1584(s)
1805(w) 1802(w)
1638(m) 1537(s)
:Ex 18%7(w)
2472(w)
2847(w) 2605(w) 2552(m)
CS
1945(w) 1921(w) 1870(w)
2472(w)
3077(m) 3054(m)
.3072(m) 3051(m)
salicylates Rb
metal
3
NO,
a’ fundamental, COOH ~rnup
a’ fundamental,
antiaym.
a’ fundamental, a’ fun~ment~l, antisym. CO,-
see text
v(CC),
v(W),
str. (a’)
vlSb
vlOa
v(W), vae @.X), vBb atr. (a’)
v(OE) v(CH)
a’ fundamental, a fundamental,
salicylio acid, and the alkali metal salicylates
K
of alkali
14.56(s)
~o~t~~.
Infra-red
3071(m) 3051(m)
Na
1487(s)
WXTTEK,
1583(%)
1%11(m) 1578(w)
Li
1485(s)
H.
1745(O) 1%x3(9)
acid Raman (solid) [331
1754(w) 1659(a)
1988(w)
Salicylio M&&d
spectra of o-nitrophenol,
1486(s)
1536(s)
::::I:~ 1684fsh)
E$:{ 1855(w)
1984(w)
2663(w)
2715(w)
2923(w)
3030fsh)
3102(w)
..~ %g;
o-Nitrophenol IMW&d RamaIl Cl41
Table 6. Correlation and assignment of the vibrational
_(
L
g tF P
it
(br.)
36
broad;
br.)
other
665(s,
E1mm] 747(s) 696(w)
870(m)
958(w)
::;471!$ 1094(w) 1080(m) 1046(w) 1028(m) 989(w)
1182(S) 1165(m)
1315(s)
symbols
821(m)
868(m)
1029(w)
1135(s)
1187(s)
1246(s)
1321(s)
o-Nitrophenol Infra-red (solid)
&B in Table
1.
670(m)
670(m)
i
809(s) 764(w)
860(m)
941(w)
981(m,
1027(w)
1082(w)
1140(m)
1200(w) 1185(sh)
1245(m)
1286(sh)
1313(s) 1299(s)
br.)
I
Infrared
1407(m) 1391(sh) 1376(s)
Na
660(S)
:::I:] 823(sh) 816(m)
1092(w)
1258(s)
1306(s)
1356(s)
1402(s)
Li
EIS”,’
875(l)
T
ALE’ 707(k)
br.)
1033(6)
1097(3)
1155(6)
1250(10)
acid Ramall (solid) [33]
853(m) 786(m) 760(B) 698(s)
963(w) 917(sh) 892(m, 868(w)
1088(w)
E%:) 1189(m) 1156(s) 1144(sh)
1251(s)
1383(m) 1325(m) 1292(s)
1404(w)
Salicylic I&a-red (solid)
668(m)
748(s) 701(m) 670(m)
812(s)
857(m)
944(w)
999(w,
1029(w)
1140(m)
1200(w)
1263(sh)
1288(sh)
1313(s) 1303(s)
1390(s)
1410(m)
Rb
aalicylates
E13 7&(“h’
br.)
I
metal
809(s)
860(m)
El?l$’
997(w,
1027(w)
::xj 1082(w)
1193(w)
1253(m)
1288(sh)
1313(s) 1294(s)
1394(s)
1410(m)
K
of alkali
Table 6-(contd.)
br.)
670(m)
;!;I:]
810(s)
856(m)
946(w)
999(m,
1029(w)
1090(w)
1142(m)
1188(w)
1254(w)
1288(sh)
:%$;j
1391(a)
1406(s)
cs I
br.:
co,-
str.
x
fundamental,
a’ fundamental,
a” fundamental, a” fundamental,
a’ fundamental,
COOII group a fundamental,
a
a’ fundamental,
a’ fundamental,
a’ fundamental,
a’ fundamental,
a’ fundamental, a’ fundamental, sym. NO, str.
sym.
see text
“ring”,
y(CH), b(CC),
v(CX),
y(CH),
?(CH),
,Y(CH),
B(CII),
d(OH)
v1
q,,(, v4
Y,~
vlOa
vllo
vob
v15
v(CO), Y,~
*(CC), “la ,S(CH), vs (a’)
(a’),
Assignment
The vibraCona1 spectra of benzene derivatives-I
[14] in carbon tetrachloride solution, are in rather better agreement with the infrared values than those of REITZ and STOCKMAIR [34], though the differences are small. There is satisfactory agreement between the present work and earlier values for salicylic acid [35-371, sodium salicylate [25] and potassium salicylate [36]. The correspondences between the spectra of o-nitrophenol and the salicylates are not as close as those between nitrobenzene and the benzoates. Although complete assignments of the observed frequencies have not been made, those given in Table 6 are consistent with such information as is available for o-disubstituted benzenes [18, 38-401. In particular, the two pairs yga, vsa and vlga, v19bof CC stretching frequencies are satisfactorily assigned as rows 16, 17 and 20, 21, respectively; the lowest CC stretching mode v14 is row 26, with the adjacent /?(CH) mode yg at row 27, if these assignments are correct for benzene. The three other b(CH) frequencies Q, vgb and vg. seem to be satisfactorily assigned as rows 32, 33 and 34, respectively. As in the benzoates, the bands in the region 1750-2000 cm-l provide some evidence supporting the assignments for the y(CH) frequencies, the values of which show some variation among the compounds. However, satisfactory confirmation of the lowest frequency vlObcould not be obtained in this way: assignment as row 45 seems reasonable [ 181, though the high intensity of the Raman line in salicylic acid is unusual. By comparison with phenol [41], the frequencies associated with the in-plane deformation of the OH group [6(OH)] and the C-O stretching vibration v,~ are expected at ca. 1190-1200 and ca. 1250 cm-l, though these modes are rather mixed. They may therefore be assigned to rows 31 and 30, respectively, but in o-nitrophenol the bands at 1266 and 1257 cm-l are probably also associated with the Y(C-0) vibrations. Some guidance in assigning the lower frequencies observed here was obtained from assignments for o-chlorophenol and o-chlorotoluene [40]. The approximately C-Cl stretching mode v,~ in these compounds is found at 834 and 803 cm-l, respectively, appearing as strong polarized Raman lines, and in Table 6 the corremode is therefore assigned to row 40. Similarly sponding v( C-NO J or v( C-CO,-) the higher out-of-plane ring deformation vq lies just above 700 cm-l in these and a number of other o&ho-compounds [18, 401 and the values of row 43 are accordingly assigned to this vibration. (In the infra-red spectrum of o-nitrophenol the absorption at 696 cm-l is extremely weak, but the band can be clearly observed, though still weak, in the spectrum of the molten substance.) Again, the “ring” vibration derived from v1 in benzene, which is at 735 cm-l in o-xylene [38], 746 cm-l in o-fluorotoluene and 749 cm-l in o-cresol[40], has dropped to 679 cm-l in o-chlorophenol and o-chlorotoluene since some substituent motion is involved. The corresponding mode in Table 6 would therefore be row 44, although the expected Raman line is not observed.
137j [38] [39] [40] [41]
A. D. H. J. K. A. J. J.
W. REITZ and W. STOCKMAIR, Mom&h. Chem. 67, 92 (1936). M. BROWN, Documentation of Molecular Spectroscopy No. 1046. Musso, Ber. de&. them. Gee. 38, 1915 (1955). LECOMTE, J. phys. radium 5, 3 (1938). S. PITZER and D. W. SCOTT, J. Am. Chem. Sot. 65, 803 (1943). R. KATRIZKY and R. A. JONES, J. Chem. Sot. 3670 (1959). H. S. GREEN. In preparation. H. S. GREEN, J. Chent. Sot. In press (1961).
Butterworths,
London
(1946).
J.H.
KCREEN,
W.K~x~s~or~and A.S.LINDSEY
In o-nitrophenol, the symmetric and antisymmetric NO, stretching frequencies are easily assigned, but the symmetric deformation frequency is not obvious. From its position in nitrobenzene and some other nitrocompounds [14] it is presumably present in the strong broad band at 665 cm-l. Similarly, the CO, symmetric deformation in the salts may be assigned as row 44, In salicylio acid the three frequencies usually associated with the COOH group [l] are at 1443 and 892 cm-l (both of which are absent in the salts) and 1292 cm-l which, however, remains in the salts as the ,8(CH) frequency Ye. (The same behaviour occurs in benzoic acid.) The spectra of the alkali metal salicylates show a number of unexpected features. Firstly, the O-H stretching frequency is absent from its expected position of about 3200 cm-l. Some di~culty was experienced in obtai~ng reproducible spectra in this region, especially of the lithium salt, where extra bands at 3300-3600 cm-l arose from the presence of moisture. But pure dry samples of the salts showed no bands at, or above 3200 cm- l. They did, however, possess a very broad band of medium intensity with sub-maxima between 2900 and 2450 cm-r. This suggests that the OH stretching frequency, which has shifted from the normal value of 3610 to 3240 cm-l in salicylic acid, has shifted lower still in the salts by reason of the strong hydrogen bonding with the anionic form [36]. The strongest bands (row 6) lie at oa. 2740 cm-l in the lithium salt and at ca. 2555 cm-i in the other salts, indicating that the hydrogen bond is weaker in the lithium salicylate, Utilizing the relationship given by LORD and MERRYFIELD [42],the hydrogenbonded oxygen-oxygen distance can be estimated at 2.82 A in salicylic acid, 2.69 A in the lithium salt and ca. 2.64 A in the other salts. These values have only a comparative significance, however, since X-ray diffraction results give, for example 2.59 A for the distance in salicylic acid [43]. A second feature is that along the series a broad band, which is absent in the acid, develops in the region 950-1000 cm-r (row 36). In the lithium salt this appears only as two weak bands, in the potassium salt it is likewise double and at a rather higher frequency but in the other salts it appears as a broad band centred at about 999 cm-l. It is similar in appearance to the y(OH) frequency of carboxylic acid dimers [I] which is found at 892 cm-l in salicylic acid. Evidence from combination bands rows 8 and 9, suggests that the y(CH) frequency Y,is underneath the broad band; no similar feature is present in the spectrum of o-nitrophenol. A pattern of five strong bands is observed in all the salts (rows 39,40, 42, 43) of which the strongest (42) is assigned as a y(CH) frequency. Near this band in lithium salicylate is a very pronounced shoulder at ea. 776 cm-l and, although much reduced in intensity, it is still detectable at 766 cm-l in the potassium and sodium salts. (This feature was completely missing in the spectra of moist samples of the lithium salt which revealed an entirely new, strong band at 790 cm-l, whilst the shoulder at 823 cm-r in the dry salt became entirely separate yielding two sharp strong bands at 812 and 822 cm- l; these changes were without parallel in the other salts.) The antisymmetric CO, stretching frequency in the salts can be satisfactorily R.C. LORD and R.E.M~RYF~~,~.C~~~. Phys.21, I66 (1953). [43]w. &%LLEE, J. mys. Chem es, 1705 (1939).
[42]
500
The vibrational spectra of benzene derivatives-I
assigned as row, 18 though in three of the compounds it is not resolved from the adjacent Y(CC) mode us,,. The frequency of the symmetric stretching mode is less certain, however. It is certainly associated with some, or all, of the frequencies correlated in rows 23 and 24. Two alternatives seem possible: (a) That the symmetric mode is row 23, and therefore virtually constant in all the salts at about 1407 cm-l. The frequencies of row 24 may possibly then be correlated with those of row 25 and attributed to a combination tone. (b) That the symmetric stretching mode is row 24, and therefore increases through the first three members of the series. Row 23 may then be attributed to a Table 7. Raman spectra of crystalline hydrated Row no. *
I K( +$H,O)
4 16 17,lS 21 23 24(?) 26 30 31 32 33 34 39 40 42 43
salicylstes [44]
-
-
7
Ca(+2&O)
I
W +2H,O)
ag( i 4&C)
-
Assignment *
.-
-
1589(2) 1471(O) 1404(& 1354( 1) 1305(l) 1261(l) 1223(2) 1151(l) llOO( 1) 1039(l)
1626(6) 1568(3)
1618(l) 1560(l)
3054(&) 1622(&?) 1580(8)
NH) v(CC) Y(C.C); anti sym. CO, str.
1496(7) 1408(7) 1361(i) 1311(4)
1473( 1) 1386(3) 1342(O) 1305(O) 1243(3) 1229(5) 1160(2) -
1486(3) 1410($) 1344(8) 1299(6) 1244(S) 1222(g) 1140(6) -
v(CC)
1036(3) 878(l) 824(4) 768(&?)
1037(6) 867(4) 818(6)
1238(4) 1164(5) 1096(&) 1041(6) 883(i) 820(5)
811(l) -
706(i) -
1 --__--l
1
, i I
sym. CO,
str.
v(CC) r(CC) a(CR) B(CR) B(CR) B(CH) Y(CR) V(CX) 1/(CR) #(CC)
* cf. Table 6.
combination frequency, most probably 2vq (row 43). A clear choice between these two possibilities does not seem possible. Evidence from the Raman spectra is not conclusive, but the frequencies observed by KAHOVEC and KOHLRAUSCH [44] are given in Table 7. These relate to the hydrated crystalline salts and the numbered rows relate the frequencies and assignments to those of Table 6. Both alternatives for the symmetric stretching frequencies are summarized in Table 5 and in spite of the uncertainty certain generalizations can be made in comparing the salicylates and benzoates: (a) For all the alkali metals the antisymmetric stretching frequency is significantly higher in the salicylates than in the benzoates. (b) The separation Av is certainly higher in the salicylates than in the corresponding benzoates. Further, the trend in the series of salicylates does not show the steady increase of value observed for the benzoates: it is either approximately constant, or decreases. [44] L. KAHOVEC
and K. W. F. KOHLRAUSCFI,Momtsh. Chmn. ‘74, 333 (1943).
501
J. H. S. GREEN, W. KYNASTON and A. S. LINDSEY
(c) The symmetric stretching frequency is lower in lithium salicylate than in lithium benzoate whilst, at the other end of the series, the values for rubidium and caesium salicylates are higher than those for the corresponding benzoate. It is clear, therefore, that variation of the metal atom in the alkali metal benzoates and salicylates causes definite and systematic shifts of frequencies in the region considered, but these shifts are not related in any simple way to the polarizability or electronegativity values of the metals. They are most likely to be the resultant of some or all of the factors mentioned above. The observations for lithium salicylate indicating a weaker hydrogen bond are consistent with the much lower degree of ionization of this salt in the solid state [45]. Acknowledgement-We benzene.
thank Mr. H. M. PAISLEY for taking the infra-red spectrum
[45] S. WIDEQVIST, Ark& Kemi 7, 229 (1954).
502
of nitro-
The vibrational spectra of benzene derivatives-I Nitrobenzene, the benzoste ion, alkali metal benzoates and salicylates J. H. S. GREEN,
W. KYNASTON and A. S. LINDSEY National Chemical Laboratory, Teddington, Middlesex (Received 8 January 1961)
Abstract-Infra-red and Raman spectra have been recorded for nitrobenzene and a complete vibrational assignment is given. The infra-red spectra of the alkali metal benzoates have been recorded and, together with the Raman spectra data, a substantially complete assignment is given for the benzoate ion. The infra-red spectra of the alkali metal salicylates, salicylic acid and o-nitrophenol have been measured, correlated and partial assignments made. Examination of the difference Av between the symmetric (v.) and antisymmetric (v,,)COz stretching frequencies in the two series of salts shows that, whereas for the benzoates Av increases steadily from lithium to caesium, for the salicylates, with the exception of the lithium salt, it is either constant or decreases slightly; v,, is 20-30 cm-l higher in the salicylates than in the corresponding benzoates; v, is lower in lithium salicylate than in lithium benzoate, but in rubidium and caesium salicylate V, is higher than in the corresponding benzoate. No simple linear relationship is found between Av and the polarizability of the free ions or the electronegativities of the alkali metals.
Introduction the infra-red spectra of organic carboxylic acids have been examined in some detail [l-3] the spectra of the metal salts of such acids have received much less attention. The effect of the metal ion on the spectra is of interest and recently attempts have been made to correlate band shifts with various properties such as ionic deformalities, ionic radii and electronegativity values [4-61. The infra-red spectra of the alkali metal salts of benzoic and salicylic acids, available from earlier work [7] have therefore been studied in more detail to investigate the frequency changes arising from changes of the alkali metal. To attempt a complete assignment for the benzoate ion, the Raman spectra of aqueous solutions of lithium and sodium benzoate have also been recorded. In view of the close similarity between the assignments for the isoelectronic pairs CH,NO,, CH,CO,and CF,NO,, CF,CO,-, a study of the vibrational spectrum of nitrobenzene was made and used to clarify the assignments for the alkali metal benzoates. Similarly, a comparison of the spectra of o-nitrophenol and the alkali metal salicylates was helpful in making the assignments for these compounds. ALTHOUGH
Experimental (a) Materials Nitrobenzene
was purified by vacuum
distillation
and examined
immediately
[I] D. HA&I and N. SHEPPARD, PTOC.Roy. Sm. (London) A 216,247 (1953); S. BRATO&D. HADZI and N. SHEPPARD,Spectrochina.Acta 8, 249 (1956). [2] M. ST. C. FLETT, J. Chem. Sot. 962 (1951). [S] F. GONZ~~LEZ-SANCHEZ, Spectrochim.Acta 12, 17 (1958). [4] R. THEIMERand 0. THEIMER,Monatsh. Chem. 81, 313 (1950). [5] R. E. KAQARISE,J. Phys. Chem. 59, 271 (1955). [S] B. ELLIS and H. PYSZORA,Nature 181,181 (1958). 171 J. L. HALES, J. I. JONESand A. S. LINDSEY, J. Chem. Sot. 3145 (1954). 486
The vibrational spectra of benzene derivatives-I
after a further distillation. o-Nitrophenol was recrystallized from alcohol-water and dried, and had m.p. 46~0°C. The lithium, sodium and potassium benzoates, and sodium and potassium salicylates were reagent grade materials (B.D.H. Ltd.). Rubidium and caesium benzoates were prepared by neutralization of AR benzoic acid with standard lithium salicylate by neutralization of rubidium or caesium hydroxide solution; salicylic acid with standard lithium hydroxide; and rubidium and caesium salicylates by adding the carbonate solution to a known quantity of salicylic acid dissolved in methanol; in all cases followed by evaporation to dryness. All the salts were dried at 110°C. Analysis for the metal gave the following. Benzoates, found: Li, 5.4; Na, 16.0; K, 24.4; Rb, 41.3; Cs, 51.5; talc. for Li, 5.4; Na, 16-O; K, 24.4; R’b, 41.4; Cs, 52.3%. Salicylates, found: Li, 4.9; Na, 14.4; K, 22-l; Rb, 38.5; Cs, 48.9; talc. for Li, 4.9; Na, 14.4; K, 22.2; Rb, 38.4; Cs, 49.2%. (b) Spectra The Raman spectra of nitrobenzene and of saturated aqueous solutions of lithium and sodium benzoate were obtained using a Hilger FL1 Raman source and E612 spectrograph, and were recorded photoelectrically and photographically to confirm certain weak bands. Infra-red spectra of nitrobenzene, and of lithium and sodium benzoates as mulls in Nujol, were taken on a Grubb-Parsons GS 3 instrument. The other compounds were examined as mulls in Nujol and in hexachlorbutadiene and, in some cases, as disks in potassium chloride using a double-beam grating spectrometer [8].
Results and discussion Nitrobenzene Nitrobenzene is not planar [9] but the departure from C,,-symmetry will not be considerable. As with aniline [lo], many of the phenyl ring modes behave according to the rules of this point group for which the thirty-six vibrations would comprise 13a, + 4a, + 12b, + 7b,. The modes additional to the normal thirty for a monosubstituted benzene are the NO, symmetrical stretching and deformation (a-J, the torsion about the C-N bond (a,), the NO, antisymmetrical stretching and in-plane rocking of the NO, group (b,) and the out-of-plane rocking (b,). For C,-symmetry with the plane of symmetry perpendicular to the phenyl ring, the a,- and b,-classes combine to give 20a’ vibrations and the a2- and b,-classes similarly give 16a” vibrations. Partial assignments have been made by earlier workers [ll, 121. The vibrational frequencies are given in Table 1 where the infra-red spectrum of the vapour [ 131 is included, since in some cases the band contours provide valuable additional information, but the region 2000-3000 cm-l is omitted since several possibilities exist to explain the combination bands found there. The present work confirms the presence of a weak Raman line [ll] at 252 cm-l and a very weak line [14] at ca. 420 cm-l, corresponding to the weak band at 420 cm-l in the infra-red [Sl [9] lo] 111 121 131 141
J. L. HALES, J. Sci. In&. 36, 264 (1959). K. E. REINERT, 2. Naturforsch. 15~1, 85 (1960). J. C. EVANS, Spectrochim. Acta IS?428 (1960). H. WITTEK, 2. physik. Chem. (Lezpig) B 52, 315 (1942). LandoltBGrnstein Zahlenwerte und Funktionen Bd. I, Teil 2. Springer, Berlin (1951). R. MECEE, Documentation of Molecular Spectroscopy No. 3368. Butterworths, London (1958). J. BEHRINGER, 2. Elektrochem. 62, 544 (1958).
487
J. H. S. GREEN, W. KYNASTON and A. S. LINDSEY Table 1. Vibrational Raman Liquid (cm-l) *
Liquid
spectrum of nitrobenzene
-
Infra-red Vapour [ 121
Assignment t
-_ 3081(m)
(0.46) (2000-3000
c
3096(m) 3068(m) -l region omitted) 1961(w) 1923(w) 1908(w) 1876(w) 1842(w) 1808(w) 1794(w) 1770(w) 1742(w) 1733(w) 1707(w) 1675(w) 1616(w)
3086( 1)
1597(s) 1586 1575(w)
0.53
1603(s) 1585(s)
1610(3)
1523(m)
0.92
1527(vs)
1550( 10)
1476(m)
0.62
1475(s) 1453(sh)
1486(3)
1412(m) 1380(w) 1358(w) 1345(vs)
dP
1412(m) 1379(sh)
dP (?) 0.19
1351(vs)
1357(10)
1316(s) 1307(s)
1309(2)
al and b, fundamentals
977 + 977 = 1954(A,) 990 + 935 = 1925(A,) 934 + 977 = 1911(B,) 934 + 934 = 1868(A,) 837 + 990 = 1827(B,) 837 + 977 = 1814(A,) 794 + 990 = 1784(A,) 794 + 977 = 1771(B,) 1069 + 677 = 1746(A,) 794 + 935 = 1729(A,) 1002 + 704 = 1706(A,) 837 + 837 = 1674(A,) 1002 + 612 = 1614(B,); 935 + 677 = 1612(B,) a, fundamental b, fundamental 793 + 793 = 1586(A,); 852 + 710 = 1562(A,) b, NO, antisymmetric stretching fundamental a1 fundamental 612 + 852 = 1464(B,); 612 + 837 = 1449(A,) b, fundamental 532 + 852 = 1384(B,) 678 + 678 = 1356(A,) al NO, symmetric stretching fundamental b, fundamental b, fundamental 610 + 678 = 1288(B,) 252 + 1000 = 1252(B,) 611 + 611 = 1222(A,)
1283(w) 1247(m) 122O(vw)
1216(w) 1172(s)
P
1174(s)
1161(s)
P
1161(s)
1108(s)
0.25
1107(s)
1074(m)
P
1244( 1) 1183(2) 1176(2) A 1171(2) 1 116O(vw sh)
a1 fundamental b, fundamental n1 fundamental 397 + 704 = llOl(B,) 397 + 678 = 1075(A,)
1094(s)
-
(w) = weak; (m) = medium; (8) = strong; (sh) = shoulder; (v) = very. * Quantitative depolarization data from [ 111. t Species designations as for C,, symmetry, cf. Table 2.
488
--
The vibrational spectra of benzene derivatives-I Table 1-(co&.)
Raman (cm-l) * Liquid
Liquid
Infra-red Vapour [ 121
1069(s)
1053(w)
1021(s)
P
1004(s) 988(w)
0.11
852(s)
0.23
793(m) 710(w) 678(w) 610(s) 532(m) 435(w) 397(m) 252(w) 176(m)
0.72
1020(s) 1002(m) N99O(VVW) 977(w) 935(s) 852(s) 837(vw ah) 794(s) 704(s) 677(s) 612(w) 532(m) 420(w)
dP (‘I) 0.87
1065( 1) 1069( 1) B( ?) 1074( 1) 1 1030(2) 1025(2) A 1018(2) 1
933( 1) C(P) 857(5) 852(5) A 847(5) I 792(4) c 701(8) C 682(sh) 591(l) 519(O)
P dP
Assignmentt
5, fundamental
al fundamental a1 b2 as b,
fundamental fundamental fundamental fundamental
a, fundamental
b 2 fundamental b, fundamental b, fundamental a1 fundamental b, fundamental b, fundamental b, and b, fundamentals a1 and as fundamentals b, fundamental b, fundamental
-
spectrum of the liquid. WITTEK [ 1 l] reported two Raman lines at 512 and 535 cm-l but other work and the present results reveal only one line corresponding to the infra-red band in the liquid at 532 cm-l, the value for the vapour being 519 cm-l. The band is definitely unsymmetrical, however, as is that recorded by MECKE [ 15]; his reported bands at ca. 410 and 450 cm-l presumably correspond to the values 397 and 420 cm-l of Table 1. Other tabulated values are in good agreement with previous measurements. Assignments of all the observed frequencies are made in Table 1 on a basis of C,, symmetry and the fundamentals are collected in Table 2, where the approximate description of the modes is that of WHIPBEN [16] and the numbering of the corresponding benzene frequencies is that of WILSON [ 171. Only the usual rather arbitrary assignment of the CH stretching frequencies is possible. The a,-component of the pair of CC stretching frequencies derived from Q, in benzene is readily assigned at 1475 cm-l and the b, component might be assigned as the weak shoulder at 1453 cm-l, but the band at 1412 cm-l with a corresponding depolarized Raman line of the same value seems more likely. This frequency is therefore somewhat lower than in other monosubstituted benzenes. The remaining “characteristic” frequencies in the $- and &-classes [ 181 are readily [15] [16] [17] [ 181
R. MECKE, Documentation of Molecular Spect~08c~pyNo. 2409. Buttmworths, London (1957). D. H. WHIFFEN, J. Chem. Sot. 1350 (1956). E. B. WILSON, Phys. Rev. 45, 706 (1934). R. R. RANDLE and D. H. WHIFFEN, Molecular Spectroscopy p. 111. Institute of Petroleum, London (1955).
489
J. H. S. GREEN, W. KYNASTON and A. S. LINDSEY Table 2. Assignment
of fundamentals
for nitrobenzene and the benzoate ion
Mode [16] and No. [17]
a’$
al
ax
20a 2 13 8a 19a
/VW
18a 12 7a 6a 1
NO,, NO,,
NO,,
Y(CH) IWH) ZJ(CH) WV X-sens. X-sens. CO, rock
b1
NO,, NO,, a2
v(CH) v(CH) +(CH) v(CC) v(CC) BWH) ring X-sens. X-sens. X-sens. CO,- sym. stretch CO, sym. deform
5s
_
9a
20b 7b 8b 19b 14 3 ;:EI; 9b 15 /WW 6b a(CCC) X-sens. 18b CO, antisym. stretch CO, rock 17a 10a 16a
Benzoate iont
3096 3081 3081 1603 1475 1174 1020 1002 1107 852 397 1351 677
3088 3073 3073 1601-1609 1483-1513 1172-1181 1015-1026 998-1006 1100-1107 828-846 404 1380-1427 673-678
990 935 794 704 176 420 532
-990 919-940 812-819 705-720 170 418-422 526-543
5 17b lob 4 11 16b
WH) v(CH) 4CC) 4CC) v(CC)
Y(CH) Y(CH) WC)
Nitrobenzene*
3081 3068 1585 1412 1316 1307 1161 1069 612 252 1527 420 or 397$ 977 837 397 5
3073 3036 1593-1597 1407-1427 1305-1314 1270-1282 1158-1163 10641073 617 1552-1561 404 5 973-984 815-840 404 $
* Liquid state values. 7 Ranges of observed values, Table 3. # Species designations assuming Ca,-symmetry, with plane of symmetry perpendicular to that of phenyl ring. 8 Frequency used again.
assigned. All the X-sensitive vibrations in the a,-class involve some CN stretching; they are assigned on a basis of the polarization and band contour data and are (That at 852 cm-l has previously consistent with the values for related compounds. [19] been identified as the CN stretching mode.) The a2 out-of-plane ring deformation v16, is almost certainly around 400 cm-l and in nitrobenzene lies at the same [I91 R. R. RANDLE and D. H. WHIFFEN, J. Chem. Sot. 4153 (1952).
490
The vibratioml spectra of benzene derivatives-I
frequency as the lowest +-mode. A detailed analysis of the combination bands in the region 1675-1960 cm-r in terms of the out-of-plane CH deformations y(CH) modes is given in Table 1 and confirms [20] the assignments made for these frequencies, all of which seem to be observed in the present spectrum of the liquid. Of the frequencies arising from the nitro group, the symmetric and antisymThe symmetric deformation fremetric stretching modes are readily assigned. quency is expected in the range 650-700 cm-l and is assigned here to the strong infra-red absorption at 677 cm-l, in agreement with BEHRINGER [14]. The corresponding Raman line is weak and recorded as depolarized [ 1 l] but with a query and we feel justified in ignoring this. That the strong infra-red band at 704 cm-l is the b, out-of-plane ring deformation Y* is shown by the C-type contour appearing in the vapour spectrum, with the 682 cm-l band present on one side of it. The out-ofplane rocking mode of the NO, group is expected to have a higher frequency than the in-plane mode because, (a) the moment of inertia of the group about an axis perpendicular to its plane is much greater than that about an axis in its plane [21], and (b) the repulsion in the potential energy function for the out-ofplane mode is probably greater than that for the in-plane mode [22]. Therefore the band at 532 cm-l is taken as the out-of-plane rocking mode; the in-plane mode is evidently at the same frequency as some other mode, probably at 420 or 397 cm-l. (In trifluoronitromethane, however, the two modes are apparently both at 400 cm-1 [23].) No evidence is available concerning the a,-torsional mode.
The infra-red spectra of the solid alkali metal benzoates (Fig. 1) are summarized and correlated in Table 3, together with the Raman frequencies observed for saturated aqueous solutions of sodium and lithium benzoates. These latter values are in reasonably good agreement with those reported by FXNKEL’SHTEIN and SHORYGIN 1241, but in the present work neither the lines at 463 and 633 cm-’ reported by them with a query, nor their weak line at 497 cm-r were observed. They reported a line at 1142 cm-l compared with the present value of 1134 cm-r: this line is anomalous and does not appear in the infra-red spectra of the salts and is absent in nitrobenzene. The infra-red spectrum of sodium benzoate is in satisfactory agreement with earlier work [25-271 but the present results show more detail. All the spectra show three aromatic Y(CH) bands unchanged with variation of the metal ion. Indeed, the expected parallelism between nitrobenzene and the benzoate ion is very close, especially in the region 800-1300 cm-l, and extends to some of the combination bands. In spite of the absence of band contour and depolarization data, an assignment, complete apart from one frequency, can be __._.. [20] D. H. WHIFFEN, Spectrochim.Acta 7, 263 (1955).
[21] A. J. WELLS and E. B. WILSON, J. C’hem. Phys. 9, 314 (1941). [22] K. ITO and H. J. BERNSTEIN, Can. J. Chem. 84, 1’70(1956). [23] J. MASON and J. DUNDERDALE, J. Chew&.Sot. 759 (1956). [24] A. I. FIN~EL’SHTEIN and P. P. SHORYOIN, D0kkz.d~ A&d. Ncmk S.S.S.R. 73, 759 (1950). 1251C. DWAL, J. LECOMTEand F. DOUVIL~, Arm phys. 17,5 (1942). [26] R. MEG=, ~0~~~~~~~ ofMolmulw S~~t~~~#~ No. 4823. Butterworthe, London (1959). [27] M. DAVIES md R. L. JOXES, J. Chem. Sm. 120 (1954). 2
491
-I**‘<*
1500
Fig. 1. Infra+A
f
’
moo qoo cm
BOO
xx)
spootra of &caXi metal benzoatay: (a) lithium, (b) LW.%ZZ~~ (c) potassium, (d) rubidru, (0) OSMS~UW
492
The vibrational spectra of benzene derivatives-I
Table 3. Vibrational snectra and as&nment of alkali met& benzoates Infra-red
Raman*
Av
Li
(cm-l) 3074
1544(m)
1392(s)
1300(w)
1155(m) 1134(m) 1023(m) 1001(s) 843(m)
617(m) 404(w)
3090(w) 307 1(w)
CS
3088(w)
1903(w) 1805(w) 1773(w) 1621(w)
191 l(w) 1800(w) 1775(w) 1621(w)
1618(w)
1615(w)
1602(s) 1597(sh) 1561(s) 1523(w)
1595(s) 1 1552(s) 1520(sh)
1594(s) 1 1552(s) 1624(sh)
1609(sh) 1595(s) 1555(s) 1527(m)
160l(sh) 1593(s) 1553(s) 1527(m)
1489(w) -
1488(w)
1502(w)
1513(w) 1440(w)
1483(w) 1447(w)
ilEl”{ 3036(z)
1427(s) 1406(s)
1413(s)
1314(w) 1304(w) 1282(w) 1237(w) 1181(w) -
1305(w) 13Ol(sh) 1270(w) 1236(w) 1180(w) Nl150(w) 1101(w) 1065(m) 1026(m) 1006(w) 974(w) 919(w) 845(m) 840(m) 819(m)
1107(w) %[:j lE~~1 940(w) 846(m) 828(m) 727(s) 720(s) 692(m) 678(m) 543(s) 422(m)
709(s)
683(m) 680(m) 617(w) 526(m) 418(m)
I
:%;:I 1926(w) 1797(w)
1407(s) 1395(s) 1384(s) I
14ll(sh) 1386(s) 1347(w)
1380(s) -
1309(w)
1311(w)
1270(w) 1231(m) 1174(w) .~1161(w) 1100(w) 1065(m)
1280(w) 1222(w) 1176(w) 1156(w) 1105(w) 1065(m) 1016(m) 1000(w) 973(w) 830(m) 812(w) 73l(sh) 719(s) 715(s)
819(m) 713(s) 705(s) 686(m)
- .. a, and b, fundamentals 2v17&41) $‘!3+ %&‘U %?O + %,b(&) r1o.xi- vl,a(Alf Blob+ v,W,f Vie -!- v&B& v172t -I- CO, sym. def. (B,} at fuudamental, rsa b, fundamental, rgb antisym. CO, str. (6,) vba + CO, sym. def. (A,) a, fundamental, visa
1791(w)
-
:E;m’ 970(Z) 920(w) 836(m)
Assignment f
3096(w) 3073(w) 3043(w) I 1979(w) 1927(w)
3088(w) 3073(w) 3036(w) 1954(w)
1973(w) 1927(w) 1823(w) 1793(w) 1619(m) 1600(s)
Rb
K
Nrt
v6b
-t
vaa(B,);
V@ + vloa(-4~) b, fundamental, vlOb sym. CO, str. (ai) 2 x CO, sym. def. (-4,) b, fundamental, ri4 b, fundamental, vQ
1280(w) 1238(w) ~1172(W) 1105(w) 1064(m) 1015(m) 998(w) -
2rse(A,f a, fundamentrtl, vpo 6, fundamental, vpb see text ox fundamental, v,* b, fundamental, vxs a, fundamental, visa a, fundamental, vl$ a, fundamental, vlTo b, fundamental, vlra ai fundamental, vBo oB fundamental, vlaa b, fundamental, vloB
828(m) 815(w) 731(w)
b, fundamental, v4 a, f~d~ment&l
678(m) I
b, fundamental, vgb 6, fundamental b, fundamental, vlab (t2 and b, fundamen-
tals, vlao
170(m)
-
b, fundamental, vll
* Obteined from saturated aqueous solutions of sodium and lithium benzortte. t Notation as for nitrobensene, cf. Table 2.
493
J. H. S. GREEN, W. KYNAS~ON and A. S. LINDSEY
made of the fundamentals and is correlated with that of nitrobenzene in Table 2, where the range of observed frequencies is shown for each fundamental. No explanation for the 1134 cm-l Raman line can be found in terms of observed Raman frequencies, but the following combinations seem possible: err + v~,~ (B,); viGb+ y4 (A,); CO,- rock (i-p) + yBb(A,). The analysis of the bands in the region 16152000 cm-l again provides support for the y(CH) assignments and indicates that an unobserved b,-frequency (vJ is at ca. 990 cm-l. The lowest b, I/(CH) frequency (v& is apparently some 20 cm-r higher in the benzoate ion than in nitrobenzene. Whilst the antisymmetric CO, stretching frequency remains unchanged at 1555 cm-i throughout the series lithium to caesium, the symmetric frequency Table 4. Vibrational
freauencies for nitro and cerboxvlate
NO,, CO,
NO,, CO, Sym.
1gym. def.
str.
C-N,
e~oups in some compounds
NO,, CO,
c-o
Antisym.
Stretch
NO,, CO,
str.
i-p rock
NO,, CO, o-p rock
I-
CH,NO, [21] CH,CO,[22] CF,NO, [23] CF,CO,[28] C,H,NO, C,H,CO,o-HOC,H,NO, o-HOC,H,CO,01
657 645 604 601 677 680 665( ?) 670(?)
1377 1425 1315 1435 1351 1392 1315 (a) .
1562 1578 1620 1681 1583 1560 1536 1582-1587
918 924 863 844 852 828 820 809-816
i
-
481 465 400 410 397(or 420) 4OO(or 420) -
608 615 400 437 532 52&543 -
-
shows a steady decrease, crossing and merging with the b, v(CC) mode (v& in the sodium salt; it is apparently split in the potassium salt. [This b, Y(CC) frequency is the higher possibility exists in two of the salts of 1440 assigned as in nitrobenzene; and 1447 cm-l but these frequencies are attributable to the same combinations as in nitrobenzene.] A smaller trend is observed in one of the v(C-0) modes, vrsa, which decreases steadily from 846 to 828 cm-l, together with a less certain trend in the higher a, p(CH) frequency (vea) from 1023 to 1015 cm.-i Some extra frequencies arise in the region 670-730 cm-l in a manner varying eratically between the salts. The symmetric CO, deformation frequency seems to be split in three of the salts and, except in sodium benzoate, an extra band appears just above the b, out-ofplane ring deformation v4. These are probably related to the physical state of the material, and the extra band may be a combination of a lattice mode with v4. The vibrational frequencies associated with the nitro and carboxylate groups in nitrobenzene and the benzoate ion are collected in Table 4, together with those for other similar isoelectronic pairs. Group frequency shifts in the vibrational spectra of solids may be due to a number of factors which at present can be assessed only on a qualitative basis. In the present case they may be summarized as follows: (a) Effects arising from variation of the spatial configuration of individual molecules, and ionic packing in the crystal. Ionic packing will be related normally to the cation/anion radius ratio which gives an approximate measure of the degree of shielding of the cations by the anions. [28] R. E. ROBINSON and R. C. TAYLOR, S~ectrochim. Acta Dr. R. C. TAYLOR.
494
15,
764 (1959);
private communication
by
The vibrational spectra of benzene derivatives-I
(b) Effects arising from electronic shifts due to intra- and inter-molecular Electronic shifts may arise through the polarization and crystal field interactions. inductive and electromeric effect of substituents, through electronic deformation of the anion by the cation (which can be related to the electronegativity value of the metal), through intermolecular field effects (which may be partly dependent on the degree of shielding of the cation) and through chelation effects where suitable substituents make it possible. (c) Effects arising from variations of the mass of the cation. Factors affecting the spectra of alkali metal benzoates are therefore likely to arise from variations of Table 5. Symmetric and antisymmetric
CO, stretching frequencies of the alkali metal benzoates and salicylates
1 Benzoates
Salicylates Antisym.
str.
Av
ionic packing, electrophilic nature of the group -CO,M+, electronic deformation of the benzoate anion, and mass of the cation. The assignments made above for the symmetric and antisymmetric CO, stretching frequencies show that the difference (Av) between these frequencies steadily increases through the series lithium to caesium (Table 5). STIMSON [29] has previously pointed out that this difference is always greater for the potassium than for the sodium salts of benzoic and aminobenzoic acids, and the present results extend this observation. It was also suggested that this separation is a measure of the deformability [30] of the anion by the cation. However, using the values of Table 5, plots were made of: (a) Av against the polarizability values of the free ions [31]; (b) Av against the electronegativity values of the alkali metals [32]; and (c) the symmetric frequency against the electronegativity values. None of these revealed any simple linear relationship. o-Nitrophenol,
salicylic acid, alkali metal salicylates
The infra-red spectra of o-nitrophenol, salicylic acid and the alkali metal salicylates shown in Fig. 2, and the vibrational frequencies are summarized in Table 6> where the numbered rows indicate the correlations made between the various compounds. For o-nitrophenol, the Raman frequencies, although obtained [29] M. H. STIMSON, J. Chem. Phys. 22, 1942 (1954). [30] K. FAJANS, Radioelements and Isotopes: Chemical lbvea Hill, New York (1931). [31] K. FAJANS and G. Joos, 2. Physik 23, 1 (1924). [32] W. C~IRDY, J. Chem. Phya. 14, 305 (1946).
496
and Optical Properties
Ch. IV.
McGraw-
J. H. S. GREEN, W. KYNASTON and S. A. LINDSEY
30002500 2ooo
1500
loco 900
800
7ccl
cm-’
.Fig. 2. Infrs-red spectra in the solid state of (8) o-nitrophenol, (b) ealicylic acid, and the mlicylates of (c) lithium, (d) sodium, (e) potassium, (f) rubidium, (g) caasium.
496
[33f E.
6
Chwa.
74, 271 (1943).
1466(s)
HERZ
and
1421(W)
1465(s)
1644(m) 1593(s)
1780(w)
>
1awv)
1856(W)
1940(w)
>
1468(s)
1488(s)
1464(s)
1486(s)
1507(ah)
1%33(m) 1584(s)
1805(w) 1802(w)
1638(m) 1537(s)
:Ex 18%7(w)
2472(w)
2847(w) 2605(w) 2552(m)
CS
1945(w) 1921(w) 1870(w)
2472(w)
3077(m) 3054(m)
.3072(m) 3051(m)
salicylates Rb
metal
3
NO,
a’ fundamental, COOH ~rnup
a’ fundamental,
antiaym.
a’ fundamental, a’ fun~ment~l, antisym. CO,-
see text
v(CC),
v(W),
str. (a’)
vlSb
vlOa
v(W), vae @.X), vBb atr. (a’)
v(OE) v(CH)
a’ fundamental, a fundamental,
salicylio acid, and the alkali metal salicylates
K
of alkali
14.56(s)
~o~t~~.
Infra-red
3071(m) 3051(m)
Na
1487(s)
WXTTEK,
1583(%)
1%11(m) 1578(w)
Li
1485(s)
H.
1745(O) 1%x3(9)
acid Raman (solid) [331
1754(w) 1659(a)
1988(w)
Salicylio M&&d
spectra of o-nitrophenol,
1486(s)
1536(s)
::::I:~ 1684fsh)
E$:{ 1855(w)
1984(w)
2663(w)
2715(w)
2923(w)
3030fsh)
3102(w)
..~ %g;
o-Nitrophenol IMW&d RamaIl Cl41
Table 6. Correlation and assignment of the vibrational
_(
L
g tF P
it
(br.)
36
broad;
br.)
other
665(s,
E1mm] 747(s) 696(w)
870(m)
958(w)
::;471!$ 1094(w) 1080(m) 1046(w) 1028(m) 989(w)
1182(S) 1165(m)
1315(s)
symbols
821(m)
868(m)
1029(w)
1135(s)
1187(s)
1246(s)
1321(s)
o-Nitrophenol Infra-red (solid)
&B in Table
1.
670(m)
670(m)
i
809(s) 764(w)
860(m)
941(w)
981(m,
1027(w)
1082(w)
1140(m)
1200(w) 1185(sh)
1245(m)
1286(sh)
1313(s) 1299(s)
br.)
I
Infrared
1407(m) 1391(sh) 1376(s)
Na
660(S)
:::I:] 823(sh) 816(m)
1092(w)
1258(s)
1306(s)
1356(s)
1402(s)
Li
EIS”,’
875(l)
T
ALE’ 707(k)
br.)
1033(6)
1097(3)
1155(6)
1250(10)
acid Ramall (solid) [33]
853(m) 786(m) 760(B) 698(s)
963(w) 917(sh) 892(m, 868(w)
1088(w)
E%:) 1189(m) 1156(s) 1144(sh)
1251(s)
1383(m) 1325(m) 1292(s)
1404(w)
Salicylic I&a-red (solid)
668(m)
748(s) 701(m) 670(m)
812(s)
857(m)
944(w)
999(w,
1029(w)
1140(m)
1200(w)
1263(sh)
1288(sh)
1313(s) 1303(s)
1390(s)
1410(m)
Rb
aalicylates
E13 7&(“h’
br.)
I
metal
809(s)
860(m)
El?l$’
997(w,
1027(w)
::xj 1082(w)
1193(w)
1253(m)
1288(sh)
1313(s) 1294(s)
1394(s)
1410(m)
K
of alkali
Table 6-(contd.)
br.)
670(m)
;!;I:]
810(s)
856(m)
946(w)
999(m,
1029(w)
1090(w)
1142(m)
1188(w)
1254(w)
1288(sh)
:%$;j
1391(a)
1406(s)
cs I
br.:
co,-
str.
x
fundamental,
a’ fundamental,
a” fundamental, a” fundamental,
a’ fundamental,
COOII group a fundamental,
a
a’ fundamental,
a’ fundamental,
a’ fundamental,
a’ fundamental,
a’ fundamental, a’ fundamental, sym. NO, str.
sym.
see text
“ring”,
y(CH), b(CC),
v(CX),
y(CH),
?(CH),
,Y(CH),
B(CII),
d(OH)
v1
q,,(, v4
Y,~
vlOa
vllo
vob
v15
v(CO), Y,~
*(CC), “la ,S(CH), vs (a’)
(a’),
Assignment
The vibraCona1 spectra of benzene derivatives-I
[14] in carbon tetrachloride solution, are in rather better agreement with the infrared values than those of REITZ and STOCKMAIR [34], though the differences are small. There is satisfactory agreement between the present work and earlier values for salicylic acid [35-371, sodium salicylate [25] and potassium salicylate [36]. The correspondences between the spectra of o-nitrophenol and the salicylates are not as close as those between nitrobenzene and the benzoates. Although complete assignments of the observed frequencies have not been made, those given in Table 6 are consistent with such information as is available for o-disubstituted benzenes [18, 38-401. In particular, the two pairs yga, vsa and vlga, v19bof CC stretching frequencies are satisfactorily assigned as rows 16, 17 and 20, 21, respectively; the lowest CC stretching mode v14 is row 26, with the adjacent /?(CH) mode yg at row 27, if these assignments are correct for benzene. The three other b(CH) frequencies Q, vgb and vg. seem to be satisfactorily assigned as rows 32, 33 and 34, respectively. As in the benzoates, the bands in the region 1750-2000 cm-l provide some evidence supporting the assignments for the y(CH) frequencies, the values of which show some variation among the compounds. However, satisfactory confirmation of the lowest frequency vlObcould not be obtained in this way: assignment as row 45 seems reasonable [ 181, though the high intensity of the Raman line in salicylic acid is unusual. By comparison with phenol [41], the frequencies associated with the in-plane deformation of the OH group [6(OH)] and the C-O stretching vibration v,~ are expected at ca. 1190-1200 and ca. 1250 cm-l, though these modes are rather mixed. They may therefore be assigned to rows 31 and 30, respectively, but in o-nitrophenol the bands at 1266 and 1257 cm-l are probably also associated with the Y(C-0) vibrations. Some guidance in assigning the lower frequencies observed here was obtained from assignments for o-chlorophenol and o-chlorotoluene [40]. The approximately C-Cl stretching mode v,~ in these compounds is found at 834 and 803 cm-l, respectively, appearing as strong polarized Raman lines, and in Table 6 the corremode is therefore assigned to row 40. Similarly sponding v( C-NO J or v( C-CO,-) the higher out-of-plane ring deformation vq lies just above 700 cm-l in these and a number of other o&ho-compounds [18, 401 and the values of row 43 are accordingly assigned to this vibration. (In the infra-red spectrum of o-nitrophenol the absorption at 696 cm-l is extremely weak, but the band can be clearly observed, though still weak, in the spectrum of the molten substance.) Again, the “ring” vibration derived from v1 in benzene, which is at 735 cm-l in o-xylene [38], 746 cm-l in o-fluorotoluene and 749 cm-l in o-cresol[40], has dropped to 679 cm-l in o-chlorophenol and o-chlorotoluene since some substituent motion is involved. The corresponding mode in Table 6 would therefore be row 44, although the expected Raman line is not observed.
137j [38] [39] [40] [41]
A. D. H. J. K. A. J. J.
W. REITZ and W. STOCKMAIR, Mom&h. Chem. 67, 92 (1936). M. BROWN, Documentation of Molecular Spectroscopy No. 1046. Musso, Ber. de&. them. Gee. 38, 1915 (1955). LECOMTE, J. phys. radium 5, 3 (1938). S. PITZER and D. W. SCOTT, J. Am. Chem. Sot. 65, 803 (1943). R. KATRIZKY and R. A. JONES, J. Chem. Sot. 3670 (1959). H. S. GREEN. In preparation. H. S. GREEN, J. Chent. Sot. In press (1961).
Butterworths,
London
(1946).
J.H.
KCREEN,
W.K~x~s~or~and A.S.LINDSEY
In o-nitrophenol, the symmetric and antisymmetric NO, stretching frequencies are easily assigned, but the symmetric deformation frequency is not obvious. From its position in nitrobenzene and some other nitrocompounds [14] it is presumably present in the strong broad band at 665 cm-l. Similarly, the CO, symmetric deformation in the salts may be assigned as row 44, In salicylio acid the three frequencies usually associated with the COOH group [l] are at 1443 and 892 cm-l (both of which are absent in the salts) and 1292 cm-l which, however, remains in the salts as the ,8(CH) frequency Ye. (The same behaviour occurs in benzoic acid.) The spectra of the alkali metal salicylates show a number of unexpected features. Firstly, the O-H stretching frequency is absent from its expected position of about 3200 cm-l. Some di~culty was experienced in obtai~ng reproducible spectra in this region, especially of the lithium salt, where extra bands at 3300-3600 cm-l arose from the presence of moisture. But pure dry samples of the salts showed no bands at, or above 3200 cm- l. They did, however, possess a very broad band of medium intensity with sub-maxima between 2900 and 2450 cm-r. This suggests that the OH stretching frequency, which has shifted from the normal value of 3610 to 3240 cm-l in salicylic acid, has shifted lower still in the salts by reason of the strong hydrogen bonding with the anionic form [36]. The strongest bands (row 6) lie at oa. 2740 cm-l in the lithium salt and at ca. 2555 cm-i in the other salts, indicating that the hydrogen bond is weaker in the lithium salicylate, Utilizing the relationship given by LORD and MERRYFIELD [42],the hydrogenbonded oxygen-oxygen distance can be estimated at 2.82 A in salicylic acid, 2.69 A in the lithium salt and ca. 2.64 A in the other salts. These values have only a comparative significance, however, since X-ray diffraction results give, for example 2.59 A for the distance in salicylic acid [43]. A second feature is that along the series a broad band, which is absent in the acid, develops in the region 950-1000 cm-r (row 36). In the lithium salt this appears only as two weak bands, in the potassium salt it is likewise double and at a rather higher frequency but in the other salts it appears as a broad band centred at about 999 cm-l. It is similar in appearance to the y(OH) frequency of carboxylic acid dimers [I] which is found at 892 cm-l in salicylic acid. Evidence from combination bands rows 8 and 9, suggests that the y(CH) frequency Y,is underneath the broad band; no similar feature is present in the spectrum of o-nitrophenol. A pattern of five strong bands is observed in all the salts (rows 39,40, 42, 43) of which the strongest (42) is assigned as a y(CH) frequency. Near this band in lithium salicylate is a very pronounced shoulder at ea. 776 cm-l and, although much reduced in intensity, it is still detectable at 766 cm-l in the potassium and sodium salts. (This feature was completely missing in the spectra of moist samples of the lithium salt which revealed an entirely new, strong band at 790 cm-l, whilst the shoulder at 823 cm-r in the dry salt became entirely separate yielding two sharp strong bands at 812 and 822 cm- l; these changes were without parallel in the other salts.) The antisymmetric CO, stretching frequency in the salts can be satisfactorily R.C. LORD and R.E.M~RYF~~,~.C~~~. Phys.21, I66 (1953). [43]w. &%LLEE, J. mys. Chem es, 1705 (1939).
[42]
500
The vibrational spectra of benzene derivatives-I
assigned as row, 18 though in three of the compounds it is not resolved from the adjacent Y(CC) mode us,,. The frequency of the symmetric stretching mode is less certain, however. It is certainly associated with some, or all, of the frequencies correlated in rows 23 and 24. Two alternatives seem possible: (a) That the symmetric mode is row 23, and therefore virtually constant in all the salts at about 1407 cm-l. The frequencies of row 24 may possibly then be correlated with those of row 25 and attributed to a combination tone. (b) That the symmetric stretching mode is row 24, and therefore increases through the first three members of the series. Row 23 may then be attributed to a Table 7. Raman spectra of crystalline hydrated Row no. *
I K( +$H,O)
4 16 17,lS 21 23 24(?) 26 30 31 32 33 34 39 40 42 43
salicylstes [44]
-
-
7
Ca(+2&O)
I
W +2H,O)
ag( i 4&C)
-
Assignment *
.-
-
1589(2) 1471(O) 1404(& 1354( 1) 1305(l) 1261(l) 1223(2) 1151(l) llOO( 1) 1039(l)
1626(6) 1568(3)
1618(l) 1560(l)
3054(&) 1622(&?) 1580(8)
NH) v(CC) Y(C.C); anti sym. CO, str.
1496(7) 1408(7) 1361(i) 1311(4)
1473( 1) 1386(3) 1342(O) 1305(O) 1243(3) 1229(5) 1160(2) -
1486(3) 1410($) 1344(8) 1299(6) 1244(S) 1222(g) 1140(6) -
v(CC)
1036(3) 878(l) 824(4) 768(&?)
1037(6) 867(4) 818(6)
1238(4) 1164(5) 1096(&) 1041(6) 883(i) 820(5)
811(l) -
706(i) -
1 --__--l
1
, i I
sym. CO,
str.
v(CC) r(CC) a(CR) B(CR) B(CR) B(CH) Y(CR) V(CX) 1/(CR) #(CC)
* cf. Table 6.
combination frequency, most probably 2vq (row 43). A clear choice between these two possibilities does not seem possible. Evidence from the Raman spectra is not conclusive, but the frequencies observed by KAHOVEC and KOHLRAUSCH [44] are given in Table 7. These relate to the hydrated crystalline salts and the numbered rows relate the frequencies and assignments to those of Table 6. Both alternatives for the symmetric stretching frequencies are summarized in Table 5 and in spite of the uncertainty certain generalizations can be made in comparing the salicylates and benzoates: (a) For all the alkali metals the antisymmetric stretching frequency is significantly higher in the salicylates than in the benzoates. (b) The separation Av is certainly higher in the salicylates than in the corresponding benzoates. Further, the trend in the series of salicylates does not show the steady increase of value observed for the benzoates: it is either approximately constant, or decreases. [44] L. KAHOVEC
and K. W. F. KOHLRAUSCFI,Momtsh. Chmn. ‘74, 333 (1943).
501
J. H. S. GREEN, W. KYNASTON and A. S. LINDSEY
(c) The symmetric stretching frequency is lower in lithium salicylate than in lithium benzoate whilst, at the other end of the series, the values for rubidium and caesium salicylates are higher than those for the corresponding benzoate. It is clear, therefore, that variation of the metal atom in the alkali metal benzoates and salicylates causes definite and systematic shifts of frequencies in the region considered, but these shifts are not related in any simple way to the polarizability or electronegativity values of the metals. They are most likely to be the resultant of some or all of the factors mentioned above. The observations for lithium salicylate indicating a weaker hydrogen bond are consistent with the much lower degree of ionization of this salt in the solid state [45]. Acknowledgement-We benzene.
thank Mr. H. M. PAISLEY for taking the infra-red spectrum
[45] S. WIDEQVIST, Ark& Kemi 7, 229 (1954).
502
of nitro-