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Vibrational spectra and structure of the cycloheptatriene molecule
Spectrochimicn
Acts, 1963. Vol. 19, I@. 1559 to 1566. Pergsmon
Vibrational
Press Ltd.
Printd
in Northern
Ireland
spectra and structure of the cycloheptatriene C. LA Lau and H. DE RUYTER
molecule
Koninklijke/Shell-Lahoratorium, Amsterdam (Shell Int,ernationalo Research Maatschappij N.V.) (Received
1.2December
1962)
Abstract-A comparative study of the infrared spectra of cycloheptatricno and of 7-D-cycloheptatriene has besn made in order to decide whether the cycloheptatriene carbon-ring system is planar or not,. The methylenic hydrogen atoms are found to occupy geometrically non-equivalent positions; this is only compatible with a non-polar ring system-molecular symmetry C,whore the carbon atom of the methyleno group is t)ilted out of the “plane” of the conjugated system. INTR~DUOTI~N
RECEXT work
in this laboratory [l, 21 has shown that cycle-heptatriene-1,3,5 (hereafter denoted by CHT) and derivatives undergo a thermal, intramolecular shift of hydrogen from the 7 to the 3 (or 4) position. ,5=6
\\ ‘,
4”
/H ?/ ,I ‘~\.X
11
/5---\ -
H--_krZ,.,’ H’
3\ 2-l/’
-X
This isomerization would presumably occur more easily in a bent CHT molecule (in the Dreiding model, possessing C, symmetry, a hydrogen atom of the CHB group is relatively close to t,he 3-4 double bond) than in a planar structure which has C,, symmetry. The symmetry of CH’I’ is however still a matter of controversy in the literature. DUKITZ and PAVLISG [3] found, from X-ray analysis, that CHT in the crystalline C,H,Mo(CO), possesses a non-planar structure of approximate C, symmetry: the methylenic carbon atom does not lie in the plane of the conjugated system. The analogy between limited regions of the infrared spectra of the complex and of free CHT makes t’hese authors suppose t,hat the C, symmetry alsoholds for the free CHT. The argument is not very convincing since the regions of the spectrum considered may well be not very sensitive to a subtle cha,nge in the molecular geometry. A pseudo-aromatic structure with a planar carbon-skeleton and possessing CZV symmetry was suggested by DOERIXG et ul. [4] on t,he basis of the N.M.R. spectrum of CHT. The use of the name tropilidene for C’HT I*eflects t,he supposed aromatic 111A. 1’. TER
Part
BORG,
VII. Rec.
H. KLOOSTERZIEL
tram. chim.,
and iY. FAN MET.KS, 7’he Chemistry
[2] 4. I’. TER BORG and H. KLOOSTXRZIEL, l%e Chsmist~~y OfC’!JClOlke~?t(chienr. tmv. chim., in press [3] .J.L>. DUNITZ and P. PACLING. FZeZv. Chim. Acta 43, ~7188(1960).
[4] W. T’. E. J. Am.
DOERIX;~,
C. LABER,
C%r,r/.Sot. 78, 5448
of Cycloheptatriene.
in press.
1%. VOSDERWAHI..
(1950). 15.59
N. E‘. (kUIBERLAIN
Part \?II.
Rec.
8Xld H. u. \1'ILLUMS,
1560
C. LA LAU and H. DE RUYTER
character* [4]. Infrared and raman data on CHT have been interpreted by EVANS and LORD [6] on the basis of C,, symmetry. These authors admit, however, that a “slight” displacement of the methylenic carbon out of the ring plane is certainly possible as the selection rules would not be sensitive enough to make such a slight departure apparent. In order to settle the problem of t’he symmetry we have studied the infrared spectra of CHT and of 7-D-CHT. The argument is essentially as follows. Only a single 7-monodeutero-CHT will exist if CHT has Czv symmetry. If CHT should possess C, symmetry, it would have two non-equivalent methylenic protons and hence two 7-monodeutero-CHT molecular species should coexist, which may be denoted by 7-D,-CHT ( more or less axial D) and 7-D,-CHT (equatorial). In principle ‘I-D,-CHT and ‘I-D,-CHT should each have their own set of vibrational frequencies. The most pronounced differences should, however, manifest t,hemselves in those vibrations that mainly involve the methylenic protons. To a less extent differences might be expected in those skeleton modes that involve appreciable motion of the methylenic carbon atom. EXPERIMENTAL
Infrared absorption spectra of CHT and 7-D-CHT as liquids at8about 30°C from 4000 to 650 cm-l were recorded with a Beckman IR-7 spectrophotometer and from 700 to 400 cm-l with a Perkin-Elmer Model-112 double-pass monochromator equipped with a C&r prism. The spectral data obtained are given in Table I, which also cont’ains the vibration numbers for CHT as used by EVANS and LORD [6]. Table 1. Spectral data of cycloheptatrieno CHT frequency (cm-l) 3060 3027 3015 2995
w VVS vs sh
2969 s
2884 s 2841 s
T’ibration
and 7 -D-cycloheptatriene
number [6]
‘I-D-CHT frequency (cm-l)
21 1.2.3 23
3060 3027 3015 2995
33 2x7 4
2964 s 2869 s 2232 2183 2140 2120
1938 m 1898 m
w vvs vs sh
m s ms ms
1938 m 1896 m
* Thermochemical data ha\-e been introduced in this argument. Sco Ref. [5] where other references are given. [5] K. CONROW, J. Am. Ckene. i%c. 83, 2958 (1961). [6] $1. V. Evans and R. C. LOED, J. AWL.Chen~.Sot. 82, 1876 (1960).
Vibrational
spectra and structure of the cycloheptatriene
molecule
1561
Table 1 (Contrl) CHT frequency, (cm-l)
Vibration
number [6]
1872 1853 1756 1739 1688 1662 1616 1606 1534 1499 1442 1434 1392 1352 1293 1246 1216 1191 1047 1016
m 1%’ ms m ins m w ms m w sh X’S s ms s sh m Ills ms ms
970 946 905 871 840 793
m ms s w VW s
31
743 vs 710 vvs 657 vs
34 35 36
589 ms
37
420 s, br 406 w, br
13 17
6 + 14 12 + 31
5.24 6
7 8 35 i- 36 25 26 9 28 29 10 30 11 12 17
‘I-1%CHT frequency, (cm-l) 1882 m 1860 VW 1850 VW 1783 w 1760 m 1750 ms 1696 ms 1655 m 1622 u 160.5 s 1640 m 1496 rnw 1440 ms 1435 ms 1396 x’s 1392 shd 1355 ms 1295 X’S 1276 ms 1242 mw 1215 ms 1191 ms 1113 ms 1056 ms 1018 wsh 1004 972 s 953 ms 909 8 862 s 843 wsh 791 m 781 s 738 vs 705 vvs 618 ms 610 ms 587 m 578 m 418 s, br ca. 404 w, br
The spectral regions most relevant to the present discussion are given in Figs. 1, 2 and 3.
Spectra of bi-7,7’-cycloheptatrienyl, 7 -phenyl-CHT and 3-phenyl-CHT, as liquid or in CCI, solution, were recorded in the 4000 to 650 cm-l region. Selected data from these specka will be used in the discussion.
C.
1562
LA
{AN! ;MITTANCE, O-
LAU and H. DE RUYTER %
3-
O-
3
I-
L toot 3
I
2950
I
2900
2850
2800 NUMBER,
WAVE
Fig.
The [1,“,7]. nuclear infrared
1. Cycloheptatriene
undiluted
liquid
I 2750 cm-’
0.0224
mm.
preparation of the cycloheptat,riene derivnt,ives is described elsewhere ‘i-Monodeutero-cycloheptatriene [l] was of 94% isotopic purity (mass and magnetic resonance spectrum), contaminated with 6Sb CHT (mass- and spectrum) and a trace of dideutero-CHT (mass spectrum). III.
DISCUSSION
(a) lnternul methylene vibrations CH, stretchings of CHT (Fig. 1). The only plausible interpretation of the CHT bands at 2969 and 6841 cm-l is that they represent internal stretching modes of the methylene group; but to describe them too strictly as the antisymmetrical and symmetrical CH, stretchings would not be correct in this case, as will appear later. The other strong band near 2884 cm-l is t’hought to be the first harmonic of the 1434 cm-i CH, scissoring mode. This first harmonic is slightly shifted to a [7] W. V. E. DOERING and
L. H. Ksox,
.7. Am. Chem. Sot. 79, 352 (1957).
Vibrational TRANSMITTANCE, 10
spectra
and structure
of the cycloheptatriene
molecule
1563
%
b
I 2250
Fig. 2. 7-I)-CScloheptat,riene
~mdiluted
I 2200
I 2150 WAVE
I 2100 NUMBER,
I 2050 cm-’
liquid 0.0224 mm.
higher frequency and intensified presumably through Fermi resonance with the 2841 cm-l stretching mode which itself in the absence of Fermi resonance might have a frequency of about 2850 cm-l. The 1434 cm-l band is also present in 3-phenyl-CHT but absent in 7-D-CHT, ‘I-phenyl-CHT and in bi-7,7’-CHT. These fact)s support strongly its interpretation as the CH, scissoring mode and thus also make the Fermi resonance plausible.
It should be noted that the separation of the two CH, stretchings (Av N 120 cm-l) is unusually large; in cases where only vibrational interaction is present Av usually is only about 80 cm-l. CHD stre2&ngs of 7-D-CHT. On the basis of C,, symmetry (of CHT) one expects for 7-D-CHT a single CH stretchin, * band near 2910 cm-l (i.e. (2969 + 2850)/2). No such band near 2910 cm-l is found, however, but instead two intense bands are present, at 2964 cm-l and 2869 cm-l (Fig. 2a). These must arise from CH stretchings of non-equivalent CHD groups, the structural non-equivalence of the protons indicating symmetry C,. According to the general literature the various unsaturated “CH” stretching frequencies of conjugated cyclic olefins and aromatics occur between 3100 and 3000 cm-l. In view of the
C. La
1564
‘AN: ;MITTANCE,
L.LU
and H. DE RUYTER
%
6’
iI
Ii
I
i
650
1
625
I
I
600 WAVE
575 NUMBER,
I 550 cm-’
Fig. 3 -- cycloheptatriene - 7-D-cycloheptatriene undiluted liquid 0.0224 mm nearness of the 2964 cm-l band to this region we verified that diphenylmethane, and cyclopentadiene had no bands between 3000 and 2940 cm-l but very intense between 3100 and 3000 cm-r as expected. More direct evidence that the 2964 cm-l band does not originate from t,riene from the fact that bi-7.7’~CHT and 7-phenyl-CHT although possessing the same system have no band between 3000 and 2950 cm-r.
fluorine, indene CH absorptions hydrogen comes triene hydrogen
The absence of the 2964 cm-l band together with the persistence of a composite band 285’7-2838 cm-l (average about 2850 cm-l) in bi-7,7’-CHT would imply that this compound has a conformation containing only one type-presumably axialof “aliphatic” protons.
Vibrational
spect(ra and structure of the cycloheptatriene
moloculc
1565
CHD stretchings of 7-D-CHT. The pronounced absorption pattern (Fig. 2b) the CD-stretching region (2300-2100 cm-l) is at first sight somewhat confusing*. The moderately strong band at 2232 cm-l is interpreted as the first harmonic of the 1113 cm-l fundamental which itself is also unique for 7-D-CHT. Its intensity might well be enhanced through some Fermi resonance with the strong 2183 cm-l frequency which is one of the two possible CD stretchings to be expected if CHT The CH/CD frequency ratio for these (e?) protons has the possesses C, symmetry. appropriate value 1.35,. The last two bands in the CD stretching region of ‘I-D-CHT are of almost equal intensity and situated at 2140 and 2120 cm-l. Here again one suspects Fermi resonance of the second CD stretching with presumably a combination frequency e.g. (1276 + 862 SL 2140 cm-l) of the appropriate molecular species. The nonperturbed CD stretching frequency would then be about 2125 cm-l giving a CH/CD frequency ratio of I.35 for the hydrogens at the other (a?) position. Methylene scissoring mode. Another “internal.” mode more or less similar to the one properly described as the scissoring mode in CH, (1434 cm-l) should be expected in the vicinity of 1300 cm-l for CHD [8]. With two non-equivalent CHD groups viz. CH,D, and CD,H, two somewhat different vibrations representing this mode can be expected: the prominent bands at 1295 cm-l and 1276 cm-l typical of 7-D-CHT apparently represent this mode. The fate of the frequencies assigned to the CH, wagging and twisting modes in CHT remains obscure; the extra bands for 7-D-CHT between 1300 and 700 cm-l given in Table 1 might have some relation with these. in
(b) Methylene
rocking
EVANS and LORD [6] assigned the 657 cm-l vibration of CHT to a (B,) CH outof-plane bending and that at 589 cm-l to the methylene rocking. In 7-D-CHT, however, (Fig. 3) the 657 cm-l band is absent but a new pair of bands appear close together at 618 and 610 cm-l. This frequency shift indicates that an appreciable amount of methylene hydrogen motion is involved in this vibration so that methylene rocking is a more appropriate description of this vibration. This view is further supported by our observations that a “lone” CHD group in monodeuterohexane gives rise to a pronounced band near 657 cm- l, i.e. some 10 per cent) below the well-known methylene rocking band of n-hexane. As will be discussed later the 710 cm-l rather than the 657 cm-l vibration must be t,he lowest YCH mode of CHT.
The doublet (618 and 610 cm-l) reflects the coexistence of the non-equivalent groups CH,D, and CD,H,. The 589 cm-l of CHT splits into the pair 587 cm-l and 578 cm-l for 7-D-CHT; t,he small shift in frequency of this vibration indicates that it is far less hydrogenic than the 657 cm-l mode. It must therefore be one of the ring deformation or bending modes involving motion of the methylene carbon atom, the band splitting again showing the a, e non-equivalence for 7-D-CHT. * The infrared spectrum of a mixture of approximately equal amounts of l-mono-D-, P-mono-D-, 3-mono-D- and 7-mono-D-CHT [l] showed that none of these four prominent bands originates from olefinic CD groups. [8] R. IN. JONES and C. SANDORFY, Chemical Applications BEIEGER)p. 408. Int’erscience: New York (1966).
of
Xpectroscopy.
(Edited by A.
WEISS-
1566
C. LA Lm
and H.DE
RUYTEI~
Our conclusion regarding the 657 cm-l and 589 cm-l assignment seems more in line with the general expectation regarding the I/CH modes as well; in all common cases the so-called umbrella yCH mode is known to be by far the most intense as well as the lowest frequency I/CH mode. One would therefore be inclined to identify the extremely intense band near 710 cm-l in CHT, and an equally sharp and single one near 705 cm-l in 7-D-CHT as the umbrella vibration and as the lowest frequency I/CH mode. (c) Additional
remarks
The 793 cm-l (31)vibration of CHT appears split int,o two vibrations at 791 cm-l and 781 cm-l in the spectrum of ‘I-D-CHT, one for each molecular species, in accordance with its assignment as a ring bending mode. The lower-frequency skeletal vibrations of CHT near 420 cm-l and 406 cm-l persist though broadened in the spectrum of 7-D-CHT. IV.
CoNCLUsIoWs
(1) The infrared absorption patterns of CHT and of ‘I-D-CHT in the hydrogen stretching regions of their CH, and CHD groups are not compatible with a plana,r skeleton and C,, molecular symmetry. (2) These patterns are, however, readily understood from the concept of a molecular structure with tilted CH, group-molecular symmetry G,--in which the methylene protons occupy structurally different positions, roughly axial and equatorial respectively. (3) Because of this C, symmetry there are two coexisting 7-D-CHT molecular species each possessing in principle its own vibrational spectrum. (4) The expected concomitant “splitting” of bands involving motion of the CH, groups is indeed observed for “7-D-CHT”. (5) Although no complete vibrational assignment has been attempted it has been shown that the 657 cm-l rather than the 589 cm-l vibration of CHT should be described approximately as the methylene rocking mode. At t’he same time the very intense 710 cm-l band of CHT and 7-D-CHT is assigned to the $H umbrella mode.
Acts, 1963. Vol. 19, I@. 1559 to 1566. Pergsmon
Vibrational
Press Ltd.
Printd
in Northern
Ireland
spectra and structure of the cycloheptatriene C. LA Lau and H. DE RUYTER
molecule
Koninklijke/Shell-Lahoratorium, Amsterdam (Shell Int,ernationalo Research Maatschappij N.V.) (Received
1.2December
1962)
Abstract-A comparative study of the infrared spectra of cycloheptatricno and of 7-D-cycloheptatriene has besn made in order to decide whether the cycloheptatriene carbon-ring system is planar or not,. The methylenic hydrogen atoms are found to occupy geometrically non-equivalent positions; this is only compatible with a non-polar ring system-molecular symmetry C,whore the carbon atom of the methyleno group is t)ilted out of the “plane” of the conjugated system. INTR~DUOTI~N
RECEXT work
in this laboratory [l, 21 has shown that cycle-heptatriene-1,3,5 (hereafter denoted by CHT) and derivatives undergo a thermal, intramolecular shift of hydrogen from the 7 to the 3 (or 4) position. ,5=6
\\ ‘,
4”
/H ?/ ,I ‘~\.X
11
/5---\ -
H--_krZ,.,’ H’
3\ 2-l/’
-X
This isomerization would presumably occur more easily in a bent CHT molecule (in the Dreiding model, possessing C, symmetry, a hydrogen atom of the CHB group is relatively close to t,he 3-4 double bond) than in a planar structure which has C,, symmetry. The symmetry of CH’I’ is however still a matter of controversy in the literature. DUKITZ and PAVLISG [3] found, from X-ray analysis, that CHT in the crystalline C,H,Mo(CO), possesses a non-planar structure of approximate C, symmetry: the methylenic carbon atom does not lie in the plane of the conjugated system. The analogy between limited regions of the infrared spectra of the complex and of free CHT makes t’hese authors suppose t,hat the C, symmetry alsoholds for the free CHT. The argument is not very convincing since the regions of the spectrum considered may well be not very sensitive to a subtle cha,nge in the molecular geometry. A pseudo-aromatic structure with a planar carbon-skeleton and possessing CZV symmetry was suggested by DOERIXG et ul. [4] on t,he basis of the N.M.R. spectrum of CHT. The use of the name tropilidene for C’HT I*eflects t,he supposed aromatic 111A. 1’. TER
Part
BORG,
VII. Rec.
H. KLOOSTERZIEL
tram. chim.,
and iY. FAN MET.KS, 7’he Chemistry
[2] 4. I’. TER BORG and H. KLOOSTXRZIEL, l%e Chsmist~~y OfC’!JClOlke~?t(chienr. tmv. chim., in press [3] .J.L>. DUNITZ and P. PACLING. FZeZv. Chim. Acta 43, ~7188(1960).
[4] W. T’. E. J. Am.
DOERIX;~,
C. LABER,
C%r,r/.Sot. 78, 5448
of Cycloheptatriene.
in press.
1%. VOSDERWAHI..
(1950). 15.59
N. E‘. (kUIBERLAIN
Part \?II.
Rec.
8Xld H. u. \1'ILLUMS,
1560
C. LA LAU and H. DE RUYTER
character* [4]. Infrared and raman data on CHT have been interpreted by EVANS and LORD [6] on the basis of C,, symmetry. These authors admit, however, that a “slight” displacement of the methylenic carbon out of the ring plane is certainly possible as the selection rules would not be sensitive enough to make such a slight departure apparent. In order to settle the problem of t’he symmetry we have studied the infrared spectra of CHT and of 7-D-CHT. The argument is essentially as follows. Only a single 7-monodeutero-CHT will exist if CHT has Czv symmetry. If CHT should possess C, symmetry, it would have two non-equivalent methylenic protons and hence two 7-monodeutero-CHT molecular species should coexist, which may be denoted by 7-D,-CHT ( more or less axial D) and 7-D,-CHT (equatorial). In principle ‘I-D,-CHT and ‘I-D,-CHT should each have their own set of vibrational frequencies. The most pronounced differences should, however, manifest t,hemselves in those vibrations that mainly involve the methylenic protons. To a less extent differences might be expected in those skeleton modes that involve appreciable motion of the methylenic carbon atom. EXPERIMENTAL
Infrared absorption spectra of CHT and 7-D-CHT as liquids at8about 30°C from 4000 to 650 cm-l were recorded with a Beckman IR-7 spectrophotometer and from 700 to 400 cm-l with a Perkin-Elmer Model-112 double-pass monochromator equipped with a C&r prism. The spectral data obtained are given in Table I, which also cont’ains the vibration numbers for CHT as used by EVANS and LORD [6]. Table 1. Spectral data of cycloheptatrieno CHT frequency (cm-l) 3060 3027 3015 2995
w VVS vs sh
2969 s
2884 s 2841 s
T’ibration
and 7 -D-cycloheptatriene
number [6]
‘I-D-CHT frequency (cm-l)
21 1.2.3 23
3060 3027 3015 2995
33 2x7 4
2964 s 2869 s 2232 2183 2140 2120
1938 m 1898 m
w vvs vs sh
m s ms ms
1938 m 1896 m
* Thermochemical data ha\-e been introduced in this argument. Sco Ref. [5] where other references are given. [5] K. CONROW, J. Am. Ckene. i%c. 83, 2958 (1961). [6] $1. V. Evans and R. C. LOED, J. AWL.Chen~.Sot. 82, 1876 (1960).
Vibrational
spectra and structure of the cycloheptatriene
molecule
1561
Table 1 (Contrl) CHT frequency, (cm-l)
Vibration
number [6]
1872 1853 1756 1739 1688 1662 1616 1606 1534 1499 1442 1434 1392 1352 1293 1246 1216 1191 1047 1016
m 1%’ ms m ins m w ms m w sh X’S s ms s sh m Ills ms ms
970 946 905 871 840 793
m ms s w VW s
31
743 vs 710 vvs 657 vs
34 35 36
589 ms
37
420 s, br 406 w, br
13 17
6 + 14 12 + 31
5.24 6
7 8 35 i- 36 25 26 9 28 29 10 30 11 12 17
‘I-1%CHT frequency, (cm-l) 1882 m 1860 VW 1850 VW 1783 w 1760 m 1750 ms 1696 ms 1655 m 1622 u 160.5 s 1640 m 1496 rnw 1440 ms 1435 ms 1396 x’s 1392 shd 1355 ms 1295 X’S 1276 ms 1242 mw 1215 ms 1191 ms 1113 ms 1056 ms 1018 wsh 1004 972 s 953 ms 909 8 862 s 843 wsh 791 m 781 s 738 vs 705 vvs 618 ms 610 ms 587 m 578 m 418 s, br ca. 404 w, br
The spectral regions most relevant to the present discussion are given in Figs. 1, 2 and 3.
Spectra of bi-7,7’-cycloheptatrienyl, 7 -phenyl-CHT and 3-phenyl-CHT, as liquid or in CCI, solution, were recorded in the 4000 to 650 cm-l region. Selected data from these specka will be used in the discussion.
C.
1562
LA
{AN! ;MITTANCE, O-
LAU and H. DE RUYTER %
3-
O-
3
I-
L toot 3
I
2950
I
2900
2850
2800 NUMBER,
WAVE
Fig.
The [1,“,7]. nuclear infrared
1. Cycloheptatriene
undiluted
liquid
I 2750 cm-’
0.0224
mm.
preparation of the cycloheptat,riene derivnt,ives is described elsewhere ‘i-Monodeutero-cycloheptatriene [l] was of 94% isotopic purity (mass and magnetic resonance spectrum), contaminated with 6Sb CHT (mass- and spectrum) and a trace of dideutero-CHT (mass spectrum). III.
DISCUSSION
(a) lnternul methylene vibrations CH, stretchings of CHT (Fig. 1). The only plausible interpretation of the CHT bands at 2969 and 6841 cm-l is that they represent internal stretching modes of the methylene group; but to describe them too strictly as the antisymmetrical and symmetrical CH, stretchings would not be correct in this case, as will appear later. The other strong band near 2884 cm-l is t’hought to be the first harmonic of the 1434 cm-i CH, scissoring mode. This first harmonic is slightly shifted to a [7] W. V. E. DOERING and
L. H. Ksox,
.7. Am. Chem. Sot. 79, 352 (1957).
Vibrational TRANSMITTANCE, 10
spectra
and structure
of the cycloheptatriene
molecule
1563
%
b
I 2250
Fig. 2. 7-I)-CScloheptat,riene
~mdiluted
I 2200
I 2150 WAVE
I 2100 NUMBER,
I 2050 cm-’
liquid 0.0224 mm.
higher frequency and intensified presumably through Fermi resonance with the 2841 cm-l stretching mode which itself in the absence of Fermi resonance might have a frequency of about 2850 cm-l. The 1434 cm-l band is also present in 3-phenyl-CHT but absent in 7-D-CHT, ‘I-phenyl-CHT and in bi-7,7’-CHT. These fact)s support strongly its interpretation as the CH, scissoring mode and thus also make the Fermi resonance plausible.
It should be noted that the separation of the two CH, stretchings (Av N 120 cm-l) is unusually large; in cases where only vibrational interaction is present Av usually is only about 80 cm-l. CHD stre2&ngs of 7-D-CHT. On the basis of C,, symmetry (of CHT) one expects for 7-D-CHT a single CH stretchin, * band near 2910 cm-l (i.e. (2969 + 2850)/2). No such band near 2910 cm-l is found, however, but instead two intense bands are present, at 2964 cm-l and 2869 cm-l (Fig. 2a). These must arise from CH stretchings of non-equivalent CHD groups, the structural non-equivalence of the protons indicating symmetry C,. According to the general literature the various unsaturated “CH” stretching frequencies of conjugated cyclic olefins and aromatics occur between 3100 and 3000 cm-l. In view of the
C. La
1564
‘AN: ;MITTANCE,
L.LU
and H. DE RUYTER
%
6’
iI
Ii
I
i
650
1
625
I
I
600 WAVE
575 NUMBER,
I 550 cm-’
Fig. 3 -- cycloheptatriene - 7-D-cycloheptatriene undiluted liquid 0.0224 mm nearness of the 2964 cm-l band to this region we verified that diphenylmethane, and cyclopentadiene had no bands between 3000 and 2940 cm-l but very intense between 3100 and 3000 cm-r as expected. More direct evidence that the 2964 cm-l band does not originate from t,riene from the fact that bi-7.7’~CHT and 7-phenyl-CHT although possessing the same system have no band between 3000 and 2950 cm-r.
fluorine, indene CH absorptions hydrogen comes triene hydrogen
The absence of the 2964 cm-l band together with the persistence of a composite band 285’7-2838 cm-l (average about 2850 cm-l) in bi-7,7’-CHT would imply that this compound has a conformation containing only one type-presumably axialof “aliphatic” protons.
Vibrational
spect(ra and structure of the cycloheptatriene
moloculc
1565
CHD stretchings of 7-D-CHT. The pronounced absorption pattern (Fig. 2b) the CD-stretching region (2300-2100 cm-l) is at first sight somewhat confusing*. The moderately strong band at 2232 cm-l is interpreted as the first harmonic of the 1113 cm-l fundamental which itself is also unique for 7-D-CHT. Its intensity might well be enhanced through some Fermi resonance with the strong 2183 cm-l frequency which is one of the two possible CD stretchings to be expected if CHT The CH/CD frequency ratio for these (e?) protons has the possesses C, symmetry. appropriate value 1.35,. The last two bands in the CD stretching region of ‘I-D-CHT are of almost equal intensity and situated at 2140 and 2120 cm-l. Here again one suspects Fermi resonance of the second CD stretching with presumably a combination frequency e.g. (1276 + 862 SL 2140 cm-l) of the appropriate molecular species. The nonperturbed CD stretching frequency would then be about 2125 cm-l giving a CH/CD frequency ratio of I.35 for the hydrogens at the other (a?) position. Methylene scissoring mode. Another “internal.” mode more or less similar to the one properly described as the scissoring mode in CH, (1434 cm-l) should be expected in the vicinity of 1300 cm-l for CHD [8]. With two non-equivalent CHD groups viz. CH,D, and CD,H, two somewhat different vibrations representing this mode can be expected: the prominent bands at 1295 cm-l and 1276 cm-l typical of 7-D-CHT apparently represent this mode. The fate of the frequencies assigned to the CH, wagging and twisting modes in CHT remains obscure; the extra bands for 7-D-CHT between 1300 and 700 cm-l given in Table 1 might have some relation with these. in
(b) Methylene
rocking
EVANS and LORD [6] assigned the 657 cm-l vibration of CHT to a (B,) CH outof-plane bending and that at 589 cm-l to the methylene rocking. In 7-D-CHT, however, (Fig. 3) the 657 cm-l band is absent but a new pair of bands appear close together at 618 and 610 cm-l. This frequency shift indicates that an appreciable amount of methylene hydrogen motion is involved in this vibration so that methylene rocking is a more appropriate description of this vibration. This view is further supported by our observations that a “lone” CHD group in monodeuterohexane gives rise to a pronounced band near 657 cm- l, i.e. some 10 per cent) below the well-known methylene rocking band of n-hexane. As will be discussed later the 710 cm-l rather than the 657 cm-l vibration must be t,he lowest YCH mode of CHT.
The doublet (618 and 610 cm-l) reflects the coexistence of the non-equivalent groups CH,D, and CD,H,. The 589 cm-l of CHT splits into the pair 587 cm-l and 578 cm-l for 7-D-CHT; t,he small shift in frequency of this vibration indicates that it is far less hydrogenic than the 657 cm-l mode. It must therefore be one of the ring deformation or bending modes involving motion of the methylene carbon atom, the band splitting again showing the a, e non-equivalence for 7-D-CHT. * The infrared spectrum of a mixture of approximately equal amounts of l-mono-D-, P-mono-D-, 3-mono-D- and 7-mono-D-CHT [l] showed that none of these four prominent bands originates from olefinic CD groups. [8] R. IN. JONES and C. SANDORFY, Chemical Applications BEIEGER)p. 408. Int’erscience: New York (1966).
of
Xpectroscopy.
(Edited by A.
WEISS-
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C. LA Lm
and H.DE
RUYTEI~
Our conclusion regarding the 657 cm-l and 589 cm-l assignment seems more in line with the general expectation regarding the I/CH modes as well; in all common cases the so-called umbrella yCH mode is known to be by far the most intense as well as the lowest frequency I/CH mode. One would therefore be inclined to identify the extremely intense band near 710 cm-l in CHT, and an equally sharp and single one near 705 cm-l in 7-D-CHT as the umbrella vibration and as the lowest frequency I/CH mode. (c) Additional
remarks
The 793 cm-l (31)vibration of CHT appears split int,o two vibrations at 791 cm-l and 781 cm-l in the spectrum of ‘I-D-CHT, one for each molecular species, in accordance with its assignment as a ring bending mode. The lower-frequency skeletal vibrations of CHT near 420 cm-l and 406 cm-l persist though broadened in the spectrum of 7-D-CHT. IV.
CoNCLUsIoWs
(1) The infrared absorption patterns of CHT and of ‘I-D-CHT in the hydrogen stretching regions of their CH, and CHD groups are not compatible with a plana,r skeleton and C,, molecular symmetry. (2) These patterns are, however, readily understood from the concept of a molecular structure with tilted CH, group-molecular symmetry G,--in which the methylene protons occupy structurally different positions, roughly axial and equatorial respectively. (3) Because of this C, symmetry there are two coexisting 7-D-CHT molecular species each possessing in principle its own vibrational spectrum. (4) The expected concomitant “splitting” of bands involving motion of the CH, groups is indeed observed for “7-D-CHT”. (5) Although no complete vibrational assignment has been attempted it has been shown that the 657 cm-l rather than the 589 cm-l vibration of CHT should be described approximately as the methylene rocking mode. At t’he same time the very intense 710 cm-l band of CHT and 7-D-CHT is assigned to the $H umbrella mode.