The infra-red spectroscopy of some highly conjugated systems–V. Conformation and electron distribution in some heterocyclic anhydro-bases

Spectroch~mtca Acta, Vol. 33A. pp 589 to 599.

Pergamon Press1917. Printedm GreatBritam

The infra-red spectroscopy of some highly conjugated systems-V. Conformation and electron distribution in some heterocyclic anhydro-bases P. J. Imperial

Chemical

Industries

Limited,

TAYLOR

Pharmaceuticals

Division,

Alderley

Park.

Macclesfield,

Cheshire.

(Receioed 3 June 1976) Abstract-Arguments based on isotopic substitution, solvent shifts. and intensity variation have been used to assign the double bond frequencies of some heterocyclic anhydro-bases. Their conformations have also been established, and on this basis the frequency and intensity of Y C==O and Y C=C are discussed. Departures of frequency from the predictions of the simple heterenone model [4] are shown to relate to the electronic character of the heterocyclic ring. By contrast, variations in eA (C==C) correlate with tautomeric ratio, being greater the more unfavourable this is, and with subsidiary factors such as the length of the overall conjugated system. Revised assignments are suggested for certain related compounds in the literature.

INTRODUCTION

for the dithiole derivatives studied by CAMPAIGNE and HAAF [7] (cf. [S]). A major cause of mis-assignment is the extraordinarily low frequencies at which their carbonyl and carbon double bond frequencies can occur. We show in this paper how, nevertheless, these can be identified by means of our previously published [14] criteria, and how the results can be used to throw light on the electronic nature of the parent molecule.

In previous papers [l-4] we have given tentative rules for the prediction of carbonyl and carbon double bond frequency in simple heterenones. We now explore the extent to which the same approach can be used when the heterenone moiety forms part of an aromatic molecule, as occurs, inter a/k, in the heterocyclic anhydro-bases considered here. JONES and KATRITZKY[S] have published i.r. data on I but make no comment beyond the observation that its spectrum resembles a pyridone rather than a simple pyridine. DENES et al. [6] give assignments for compounds related to VII which are almost certainly at fault (see below), and the same may be true Table

1. Solvent

EXPERIMENTAL Compounds V [9) and VI [9] were supplied by Dr. G. de W. Anderson, 1X by Dr. S. Birtwell, and VII [9], VIII 191. and X by Dr. M. S. Magson. We have

effects on the i.r. spectra of compounds

I-X*

I Nujol

cq

Dioxan

MeCN

CH,Cl,

DMSO

CH212

CHCI,

1657MS

1676MS 1638MS 1548VS

167lMS 1639s

1530MS

1528MS

1664MS 1636MS 1546VS 1527s

1264M 119lM

(1189)

1665MS 1635MS 1545vs 1525MS 1266M I l9lMW

Il90M

1661s 1632MS 1545vs 1522MS (1264) 1188MW

I659MS 1633MS 1541vs 1524s 1263M 1188M

1657 1632 1543 1523 1265 1189

390 360 I260 630 340 350

115ovs

-

I l52VS

Il5OVS

115ovs

Il49VS

I151

I250

1059s

1059

6 70

1632MS 1544s 1516s 1267MS 1200MW

1549 vs

1061s 772s SA. 3315 -E

776M

778M

775M 589

772MS

Y

c==a

v ring At \’ c===C v ring Bt 1’ co 13CN exo v CC v OEt

P. J. TAYLOR

590

II

Nujol

CCI,

Dioxan

CH&I,

CHCl,

1680s 1627MS 1576W

1682MS 1628MS

1679MS 1626MS

1673MS 1627MS 1575w 1542s ISIZMS 1496MS 1278MW 1244M 1168s 1053MS

1669 I626 1574 1539 1511 1495 1278 (1246) 1168 IO52

1553MS

1515M 1496M 1284W 125lM 1169s 1059M

(1512) 1496M 1281 W 1245MW 1168s I053M

(1543) -

-

CH2Iz

Notes 1’

X60 5/O 6 70 I IO

1666MS 1622MS 1572W 15388 1510MS 1492MS 1277MW

Y6fJ 3X0

(I 163) -

\’ ring At I’ phenyl 1’ c=C I’ ring Bt 1’ phenyl \’ co 1’ CN exo v CC 19OEt

CHCI,

Notes

3 7fJ 550

c=o

III Nujol

CH,CII

Dioxan

16683

1697M 1665sh 1646MS

1663MS 1645MS

1637MS 1596W

1606MS 1579s 1516MW 1498MW

1554vs 1530Msh l497M 1296MW 121lMS

CHzI,

1693MS

1691

410

1643s 1602w

I 640 1600

X10

157ovs

II 70

$92)

1572 1515 1495 _

1151s

_

1690M 1663M > 1641 MS 1606s

1608s

12lOMW 1172M 1151s 106IMW

1151vs 1063MS

MeCN

1574s > 1512M 1495M (1290)

530

1172s 1151s > 1064MS

\’

c=o

1’ ring At v phenyl lc=C 1’ ring Bt Y phenyl \’ co v CN exo 1’ CC v OEt

IV Nujol

cq

Dioxan

CH,CI,

DMSO

CHC&

1639s 1580M I563MS 1549MS 1488M l423M 1334M 1167s I062M

1661 MS 1585M

1654MS 1583M

1648MS I584M 1565MW 1528 VS 149lM 1430M 1336M 1167VS 1054M

lfA6MS 158lM

1644 1583 1558 1525 1489 1431 1336 1169 1053

1567sh

1560sh

1535 vs 149lM 1433MS 1335M 1166VS I057M

1531 vs (1490) (1429)

1558sh

1526VS 1488M -.. 1 l66VS

350 _7_7fJ I I)xo 300

SYfJ 260 1320

400

CHzJ2

Notes

1644MS 1583M 1557MW I524 VS 1490M 1430MS 1334M 1166VS 1053M

VC=O v phenyl ring v C=C exo I’ C=C v phenyl 1’ co/cct 1’ CN \’ cc/cot I’ OEt

V Nujol

CCI,

1624MS 159ow 15OOvs 1484s 1445sh 1351 w 1182MW 1 I36MS 956M W

1630 1592 1498 1474 1445 1352 1176 1132 952

31.5 l-70 13NfJ 1310 1560 130 130 -745 -77.5

Dioxan

CH,Cl,

DMSO

CHZI,

CHCI,

1623MS 159lW 1495vs (1472)

1617MS 1593w 1494vs 1472VS 1422M 1352VW 1166MW 1135M 953M W

1615MS 1589W 1494 vs 1469 VS

1615MS 1592w 1491 vs I470 vs 1443sh 13491/w

1612

1175MW

1178 1136 952

951M

1174w 1132M

953M W

Notes

1592

240 1h.c

\’ c=o 11ring

1490 1471

1410

1’ ring

I NfJfJ

Y c=c v ring \’ CN exo v CC

1445sh

1352

I30 /3fJ .?/fJ 240

Infra-red

of conjugated

systems-V

591

VI Nujol

CC],

Dioxan

CH2C12

DMSO

CHCI,

l592W 1493v.s 1470s 1452sh 1348VW 1290MS 1226MS 1132MW 955M

l592W 14981/S 1471s (1460) 1355w 1292M 1229MS 1134w 950M

l588W 1496VS

1594w 1495vs 1471s l45Osh 1352VW

1588VW 1494vs (1468)

1592 1493 1471 1450sh

-

1230M

1230M

1227MS

952M

948M

951MW

95 1X90 950

CH,I,

Notes

159ovw 1491 vs 1468s 1455sh

I* ring 13ring 1’ C==C r ring v CN Y C==s/cc 1’ cc/c==s

1293 1225

-7X0 3 70

1293MW 1224MS

950

1X0

951M

VII Nujol

CCl.$

1604M l572MS 150lMS 14868 1476VS 1438MS 1337M 1216MS 1138MW 948M

l612M 1577MS 1498sh 34898 1471 vs 1447MS 1336M 1215MS l142MW 946M

CH,Cl,

345 300 (1030) 1720 1860 410 200 X00 115 1X0

1606 M 1598 > 1571MS 1496MS 1485s 1468 VS 144OMS 1338MW l215MS ll42W 947M

MeCN

DMSO

16OOM 1571MS

1599M 1570MS 1496sh l482sh 1469VS

1334w 1217MS 1143MW 949M W

1218MS 1142W

CHJ,

CHCI,

1603 M 1598 > 1568MS l496MS 14848 1466VS 1438MS 1333MW l215MS

1602sh 1596 1566 1494sh 1482 1466 1434 1334 ~ 1139 945

947MW

Notes

2x5

415 (59fJ) 1530 2380 750 23(J

1’phenyl v C=o I’ phenyl v ring v C==C v Ph/ring v CN exo v CC

130 I70

VIII Nujol

CHzClz

CHCI,

1590w 1578MW 14988 34878 14668 1451MS 1332MW 1272MS 1225Mbr 1146W 956M

l594W 1580VW 1500s 1492s 1470MS 1445sh 1330MW 1231MW

1594 1580 1496 1489 1469 1443 1335 1270 (1228)

958MW

954

14963 1490vs 1470MS l450sh 1335w (1230) (1146)

70 90 (1920) 2150 11,70 250 200 SOS

DMSO

CHJ,

Notes

l588W l575W 1496MS I4863 (1466) (1448)

159ow l576W 14943 1485VS 1465MS 1440sh l335W 1267MS l230M

v phenyl v ring 1’ phenyl v ring 1’ c==C v Ph/ring 1%CN 1’ C=s/cc 1’ cc/c=s

1264MS 1230M

2 00

952M W

IX Nujol

cc14

16OOM 1572MS 1539M l5OOM 1473 vs (1462) 1439s 1354M 12248 920MS

1603 1576 1543 1504 1478 1466 1445 1357 1227 923

3MJ 430 155 455 1350 1110 970 220 70CJ 2X0

CHIClz

CHCI,

1599M 1568MS 154OMW 1500M 14741/s 1463VS 1437s l353M 12258 918MS

1597 I566 1539 1499 1472 1462 1438 1352

23.5 510 205 365 1360 13w 1140 -735

919

275

CHZIZ

Notes

1596M 1564MS 1534MW l498M 1469 VS 14583 14333 1349M 12233 917M

1’ phenyl \’ c=o 1’ C=N v phenyl v phenyl rC=C I’ phenyl v CN exo r CC

P. J. TAYLOR

592

X

Nujol

ccl,

1593vw 1538MW 1496M 1479s

1492M W 14778 1462M 1446MW 135ow 1244MW 1216W 914vw

135ovw I264M 1223W 917w * Intensities i See text. Table

I

’

II

Dioxan

1525W 1493MW 1477s

1528W 1493MW 1476s

(1257) (1223)

1343MW 1218W

CHKI,

CHCI,

1593vw 1529W 1493M 1476s 1462M 1446MW 1349w

1593 .1528 1493 1475 1461 1445 1349 1257

(1219) 914vw

CHzIz

Notes

1527W 1492M 1473s 1457M 1444MW 1346W 1256M 1218W

v phenyl v C=N v phenyl v phenyl v c==C v phenyl 1’ CN Y c=s/cc \’ cc/C==s

I25 580 I720 730 380 155 430

914

are italic.

2. Comparisons

Compound

DMSO

between

anhydro-base

v

“In

1657 1544

1601

1669 1539

1604

double

bond frequencies

(cm -’ in CHCI,)

and heterenone

Y*

“In

-N/

1660 1565

1613

-12

$oR -“ii

1660 1565

1613

-9

1635 1580

1608

27

1660 1565

1613

19

Model

Av,

models at

0.0

0.65

F&o, IIIa

I663 1606

-“w

1635

?+o IIIb

1690 1574

1632

IV

1644 1525

1585

1645 1540

1593

-8

V

1612 1471

1542

1605 1510

1558

-16

VII

1566 1466

1516

1585 1490

1538

-22

* cJ: [4]; predictions t See ref. [lo]

for VII based

-;;

on two examples

0.72

only.

reported [lo] the preparation of I-IV. Spectroscopic techniques have been described [l]; pK, values were obtained as previously [lo] and are given in Table 2. The timedependent changes shown for the cation spectrum of IX

are pH-reversible and may be due to equilibration between the cations IXa and IXb (Scheme A). It is not clear why IX alone should behave this way. Me

OMe

0.75

593

Infra-red of conjugated systems-V

P

N-EN’

PhA

Me CHCOAr CHCOPh

RESULTS

It is convenient VX, in sequence. Compounds

to discuss

compounds

I-IV,

and

I-IV

As previously [2], four planar aligmments are possible around the carbon double bond. Those with carbony1 CIS- to NMe may be rejected on steric grounds and because NMe is the best donor group so TRANS-conjugation will be favoured [2,4]. Compound III provides the decisive evidence as between

the TRANS-s-tram and TRANS-s-cis conformers. Two solvents (Table 1) show mixtures of conformers (see Fig. 2). One shows v C=O and v C=C close to 1665 and 1610cm-i, the other near 1690 and 1580 cm- ‘. On previous arguments [2,3] these pairs belong to the s-tram and s-cis conformers respectively. It is curious that the proportion of the latter rises with solvent dielectric constant since the former is more extended so should be more polar. Perhaps the s-trans form is more twisted, though since mean frequency (v,) is nearly identical the difference cannot be great [4]. The solid state is clearly in the s-cis form.

_

P. J.

594

TAYLOR

100

61

I 1800

I 1700

I

16lm

I

1540

I

1400

I r 13t-n 18fJl

I

1700

I

1600

I

1YlO

I

1400

I 1300

Vh-1

Fig. 1. Infra-red spectra of (a) compound I, (b) compound II. in CHCIS. Of the rest only IV shows both conformers, with v C==O and v C==C at 1639s and 1549 MS cm-’ in Nujol (Av 90 cm-‘) but near 1650 MS and 1530 1/S cm-’ (Av 120cm-‘) in solution (Fig. 3). Here S, the slope of v C==C vs. v C=O, is 0.60, in line with predictions for the TRANS-s-cis alignment in simpler models [2]. From S and Av the s-cis conformer follows for solution and the s-tram for the solid state. The same criteria assign I and II to the s-cis form throughout. The position is summarised in Scheme

A. For I-III, S is not a particularly useful criterion; both v C===O and 11C==C show additional coupling (see below), and band overlap between the conformers of III adds further complications. Simple aminoenones show a slight preference for the s-mm conformer [l l] which is contra-indicated here. The reasons may be steric. Alkoxyl occupies more space than carbonyl [l2] whereas hydrogen (on N) projects further than the nitrogen lone pair [13]; the s-tram conformer should therefore be more disfa-

100

/

%T

0 1600

1%X

J/cm-l

Fig. 2. Infra-red spectra of III: (a) in dioxan (D = 2.2); (b) in CHCl, (D = 4.8); (c) in CHzClz (D = 9.1); (d) in MeCN (D = 38).

595

Infra-red of conjugated systems-V 100

701

I I’

i

I’

II ”

0

I

1800

I

1

1600

1400

I 1200

I 1Mx)

I 802

V/0X-1

Fig. 3. Infra-red spectra of IV in Nujol (full line) and CHCI, (dashed line). voured in I and II, where it is never found, than in III and IV. An attraction between the hetero-atoms as in the thiathiophthens [14] is rejected as explanation since there is no evidence for anomalous carbony1 frequency lowering.

and

5I’

,”

‘4

“yring

5”

and

u,,c *c B In addition to v C==O and exocyclic v C=C, compounds I-III possess two other strong, and sometimes solvent-sensitive, bands in the 6 p region (Fig. 1). These bands, designated v ring A and v ring B in Table 1, lie at 165&1620 and 153&1510cm-’ respectively. We suggest an analysis along the lines of that by SPINNER and WHITE [15] for the 2-pyridones (see Scheme B for the comparison), the “ring A” and *We have previously [16] assigned the strong 1166 cm-’ band of IV to v,, CNC. but now withdraw this.

“ring B” bands being essentially v,, CLX and v, C=X (X=C or N) respectively. On solvent shift evidence, these tend to couple with v CL0 and exocyclit v C==C respectively; for I, a plot of this v c--C plus “v ring B” vs. v C=O plus “v ring A” is notably more linear (S = 0.55) than the standard plot [l]. For II where the standard plot is linear the above procedure produces no improvement, whereas for III the situation is confused by band overlap. In IV ring v C==C is weak, presumably because of its cross-conjugated position. The resulting competitive conjugation is presumably the reason why, as in tl, b-unsaturated ketones [1] but not in simpler heterenones [224], exocyclic v C=C suffers such a notable loss of intensity here in the solid-state s-trans conformer (Figure 3). Assignments for ester v CO and v OR, and for conjugated v CC and v CN, follow on established [3,4] lines; for III, the last is confirmed by a detailed study of the conformers. It is probable that IV shows complications due to coupling between v CO and v CC of a type previously encountered [3].* Compounds V-X For all these compounds the TRANS-s-cis alignment can be assumed (Scheme A), the steric argument being stronger than before and the rest unchanged. As above (and see Discussion), there is no reason to suspect specific through-space interactions. Compound V contains a band near 1620cm-’ which is absent from VI; this must be v c--O. Similar

596

P. J. TAYLOR 1OC

%T

ICI

0

-

16rn

13m

‘14bo

1600

1Ym

1300

I I6W

1

15al

I

I4m

I 1300

VICK’

Fig. 4. Infra-red spectra of: (a) VII in CHCI,, (b) VII in Ccl,, (c) VIII in CHCI,.

bands near 1570 cm- ’ in VII and IX are absent from VIII and X. In the latter cases there is probably some coupling with a ring band near 1590 cn- ‘, as shown by some solvent sensitivity for the latter, but the 159Ocn-’ band remains in the thione even though much weakened (see Fig. 4) so the 1570 cm-r band is essentially carbonyl. We therefore suspect the unassigned band at 157s155Ocn-’ found by DENES et al. [6] in XI, and not that at 163&1600cm-1, to be the carbonyl mode. All compounds contain three strong bands in the 1500-1450 cn- ’ region. Two of these including that near 1450cm-’ are expected to be benzenoid; the third must be exocyclic v C==C. Both bands near 1500 cm-t are solvent-sensitive, so this criterion [4] will not distinguish them. However, one band in each ketone gains intensity at the expense of the other as solvent polarity rises; this has been shown [4] to be diagnostic for conjugated v C=C, albeit heavily coupled (see Fig. 4). This band is generally the stronger of the two; in VII it is the strongest we have encountered in a heterenone. In IX and X the pattern differs to the extent that coupling (or Fermi resonance?) of v c--C is stronger with the 1450cm-’ benzenoid mode. These compounds lack the benzene ring annelation present in V-VIII; plausibly, the 1490 cm-’ band in those cases is dominated by a mode of the annelated ring, and coupling is assisted by molecular rigidity. * Since one can write a similar dipolar canonical form for cyclopropenone as for other enones[l], its C=C stretch is not strictly localised any more than here. Unfortunately, there have been no solvent studies on this compound.

Whatever analysis is accepted, the outstanding feature of these compounds is their low carbon double bond frequencies, mostly below even those for the cyclopropenones described by MILLER and his coworkers [ 171 as “. . startling. . . by far the lowest local&d C=C stretches known to the authors.“* The question arises as to how far our previous analysis [l-4] of v C==O and v C=C, as v,, C-O/C&C and v, c--C/C=0 respectively, remains valid at a separation of up to 150cm-‘. Simple mechanical coupling between two carbonyls is broken if one is substituted by thione; the other reverts to its “natural,” uncoupled position [ 163. Nothing like that happens to these v C&C on thio-substitution. Either the proposed coupling is absent, or there is a special explanation. Possibly v C==C is so low in these compounds that v C=S is not an impossible distance lower, so can replace carbonyl in this coupling. Identification of thiocarbonyl is often difficult through mixing with low-lying vibrations [18]; here we suspect coupling between v C==S and 1’ CC (elsewhere near 1200 cm- ‘) to give bands at 129&1250 and near 1220cm-i of which the former is assigned to mostly v C==S since its highest frequency is found for VI. On this basis the isotopic ratio v C-O/v C==S for these three pairs of compounds is (in CHCl,) 1.25, 1.23, and 1.23. Whether effective coupling between v C==S and v C==C can take place at a separation of 200 cn- ’ is a moot point, but indubitable examples involving v C=O and v C==C or v C==N have been found [ 191. On our previous analysis[l] one expects v, (r c--C) to lose in relative intensity with increasing solvent polarity when v,, (v C==S) is the lower frequency,

Infra-red of conjugated systems as in the 4-pyridones [2]. The evidence is slim, but in the absence of other indications, v C-------Cfor the thiones is assigned on this basis. The identification of v ~ as a band around 1500cm-~ also probably applies to XI[6]. KATRITZKYand ROWE [20] assign a very strong band at 1500cm-1 in XII to carbonyl; most probably this is v C-------C,and v ~ will be found as a weak band in the 1600-1550cm - t region. Other bands: v CN lies near 1350cm -~ as in IV rather than near 1200 cm-1 as in I - I l l ; this probably reflects the "n-excessive" and "n-deficient" [21] characters of IV-X and I-III respectively. Conjugated v CC is very constant at 1200-1160 cm-1 except when coupled to v ~ S .

DISCUSSION

One ultimate objective of this work is to provide a rationale as to why the double bond frequencies of complex heterocycles should be as they are. A start has been made with correlations for simple heter-

H

V

597

for I-IV and any discoverable electronic index, but the deviation Arm from v, for the heterenone model follows the order expected for the electron-withdrawing effect of the second (after NMe) hetero-substituent (or of Table 2). This supports vm as the true index of conjugation [2,4]; it may in fact be better than tr in this context. The exceptional rise in Av, for III is probably caused by lack of planarity, which may also explain why both conformers co-exist. The further fall in Avm from IV-V is expected in terms of the rise in electron density brought about by benzene ring annelation [21]. Intensity follows different rules. Here as elsewhere [2] carbonyl (sic) intensity, which is relatively low, appears to be randomly variable; attention concentrates on that of the carbon double bond. There is no correlation between EA (C------C)and any function of frequency, certainly for I-IV. Instead, three of these four compounds fit equation (i); EA (C-----C) = 430 - 132 pKT

the shortfall for III (Table 3) may be caused by its lack of planarity.

H

/ffff~CHCOR

___~CH2COR XI.__V

H

_/~-CHCOR

×llla

Xl l lb

+JF H+

~

(i)

H

O" I

N

-

~__j}-CH'CR I.._~lc XI

JF

H N

OH

CH2COR X__V

x-~

C enones [4]. We now use these to construct notional A simple but effective rationale is the following. frequencies which the heterenone moiety in each com- Compounds I-IV protonate on carbon [10]; that is, pound (except the thiones) should have possessed in XIII is related to the favoured tautomer XIV through the absence of conjugation with the rest of the mol- the common cation XV [22] (Scheme C; pyridine is ecule. By hypothesis, the difference will reflect its elec- chosen as model but the same argument applies elsetronic hinterland: whether net donor or acceptor in where). For XIV to be favoured over XIII means that character. The data are summarised in Table 2. aromatisation of the ring is preferred. Therefore the Six-membered ring heterocycles are n-deficient, this more XIII is disfavoured the greater the contribution tendency increasing with aza-substitution [21]. Hence of the aromatic canonical form XIIIb, from which v C.----O rises, relative to the model (and considering XV is formally derived by protonation. Hence a taus-cis forms only) in the order I < II < III. That the tomeric ratio increasingly disfavourable to XIII (pKr ring of I appears electron-rich presumably results more negative) increases both its relative basicity and from a zero-shift in the scale: N-methylation converts the intensity of vs C.------C/C-----O(v C-------C)which we an electron sink part way towards an electron source. have shown [2,4] to be linearly related to the size There is no overall relation between v C.------Oor vm of the contribution from dipolar canonical forms,

P. J. TAYLOR

598

Table 3. Data on pK~, u.v. spectra, and tautomeric ratio for some heterocyclic anhydro-bases Compound

B

BH +

pK~

pK r't

obs.

calc.

I II III IV V Vll IX

374 335 334 331 359 387 383

264 276 274 238 355 373 373 ----,364§

10.54 5.73 7.02 5.70 2.92 II 0.5411 1.63 II

- 6.39 - 3.39 - 6.73 - 4.74 ----

1260 860 1170f 1080 1800 2380 1360

1280 880 1330 1060 --

* Longest wavelength band of substantial intensity (CA > 103) for free base (B) and cation (BH ÷) in water. 5"K = (XIII)/(XIV); cf. Scheme C and ref. [10]. :1:From equation (i). § See Experimental section. II In 20% MeOH. ¶ In CH2I 2. At the core of the difference between the rules for frequency and intensity lies the distinction between absolute a n d relative energy levels. Pyrimidine is less basic than pyridine through the electron-withdrawing effect of the extra nitrogen a t o m ; this is an absolute distinction that spills over into I and II. Hence the latter has higher double bond frequencies relative to the same model heterenone. However, the loss in aromaticity brought about by N-methylation is similar for both, and therefore so is CA (C~----C). By contrast, this loss of aromaticity ( - p K T ) is much less for IV, so EA (C------C) is less despite the lower frequencies (drop in v,,) which, on simpler analogies (e.g. amides vs. esters), might have argued for higher intensity. The analysis for II is different again, but runs on similar lines. In each case, frequency relates to an absolute energy scale but intensity to a much more relative one. The ketones V X protonate on oxygen as is shown by their contrasting u.v. behaviour (Table 3), and since the cation XVI (Scheme C) does not relate to XIV, there is no way of estimating tautomeric ratio.* If however this is similar for IV and V one presumably explains the latter's higher double bond intensity as due to the more extended conjugation that results from benzene ring annelation. Its still further rise in VII can be similarly explained (phenyl can share the negative charge) and this conclusion is reinforced by the fall in CA (C~--C) for IX. In so far as a switch to cation structure XVI may be regarded as reflecting an increased importance of the canonical form XIIIc in c o m p o u n d s V X. it is tempting to regard their anomalously large further fall in double bond frequency as resulting from this * KATRITZKY et al. [23] find C-protonation for XII, but this may be a function of the high tautomeric preference for the aromatic pyridine ring.

cause. If so the frequency and intensity changes in IV, V and VII are roughly related in the conventional manner, though that is not so elsewhere. Whatever the naiveties in the above approach it does explain the lack of any general correlation between frequency and intensity shown in this series, as well as supplying a rationale for each trend separately, and it may find application in other cases. We shall explore some of these in future publications.

REFERENCES

[1] P. J. TAYLOR, Spectrochim. Acta 32A, 1471 (1976). [2] D. SMITH and P. J. TAYLOR, Spectrochim. Acta 32A, 1477 (1976). [3] D. SMITH and P. J. TAYLOR Spectrochim. Acta 32A, 1489 (1976). [4] D. SMITH and P. J. TAYLOR, Spectrochim. Acta 32A, 1503 (1976). [5] R. A. JONES and A. R. KATRtTZKY, Aust. J. Chem. 17, 455 (1964). [6] V. 1. DENES, GH. CIURDARU, and M. F~,RC~SAN, Rer. Roumaine Chim. 9, 375 (1964). [7] E. CAMr'AIGNE and F. HAAF, ,1. Org. Chem. 30, 732 (1965). [8] A. R. KArRtTZKY and P. J. TAYLOR, in Physical Methods in Heterocyclic Chemistry (Edited by A. R. KATRITZKV). Academic Press, 1971, pp. 308 et seq. [9] L. G. S. BROOKER,G. H. KEWS, and D. W. HESELTINE, J. Am. Chem. Soc. 73, 5350 (1951). [10] R. G. BUTTON and P. J. TAYLOR, ,1. C. S. Perkin 11, 557 (1973). [11] J. DAaROWSKI and L. KOZERSKI, J. Chem. Soc. (B), 345 (1971). [12] J. M. O'GoP, MAN, W. SHAND, and V. SCHOMAKER, J. Am. Chem. Soc. 72, 4222 (1950). [13] A. R. KATRtTZKY,in Topics in Heterocyclic Chemistry (Edited by R. N. CASTLE). Wiley-lnterscience, New York (1969). [14] R. PINEL, Y. MOLLIER, and N. LOZAC'H, Bull. Sac. Chim. France, 1049 (1966).

Infra-red of conjugated systems-V [15] E. SPINNERand J. C. B. WHITE, J. Chem. Sot. (B), 991 (1966). [16] P. J. TAYLOR, Specrrochim. Acta 26A, 165 (1970). [17] E. C. TLJAZON,D. H. FINSETH,and F. A. MILLER, Spectrochim. Acta 31A, 1133 (1975). [IS] L. J. BELLAMY,Adoances in Infrared Group Frequenties, Methuen, London (1968). [19] P. J. TAYLOR, unpublished observations.

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[20] A. R. KATRITZKYand T. D. Rowe, Spectrochim. Acta 22, 387 (1964). [Zl] A. ALBERT, Heterocyclic Chemisrry. Athlone Press. London (1968). [22] A. R. KATRITZKYand J. M. LAGOWSKI.Adu. Heterocyclic Chem. I. 341 (1963). [23] S. GOLDING, A. R. KATRITZKY, and H. 2. KUCHARSKA,J. Chem. Sot. 3090 (1965).