3350
The Journal of Physical Chemistry, Vol. 83, No. 26, 1979
Hirota et al.
ODMR and EPR Studies of the Triplet States of Aliphatic and Aromatic Carbonyls N. Hirota," M. Baba, Y. Hlrata, and S. Nagaoka Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan (Received August 14, 1979)
We have investigated the properties of the lowest excited triplet (T,) states of aliphatic and aromatic carbonyls
by means of ODMR and EPR techniques at liquid helium temperature. Some of our recent observations * and 3 ~ benzaldehyde ~ * are discussed. In the T1state of 2-indanone excitation concerning 3 n ~ 2-indanone is localized at the carbonyl moiety making this system as a model of ha* aliphatic carbonyl. The phosphorescence spectrum and the decay characteristic indicate a large distortion from planarity. Low field ODMR experiments confirm the highly distorted structure of the 3n7r*state 2-indanone. We discuss the sublevel decays in view of this distortion. The zero field splitting (zfs) of 3 ~ 7 r *benzaldehyde is affected significantly by deuteration of the formyl hydrogen. This effect is primarily due to the change in the spin-orbit contribution to zfs caused * by the blue shift of the 3n7r*state energy. The matrix element for the spin-orbit mixing between 3 n ~and 3 ~ states ~ * was estimated from the changes in the zfs. The spin distribution in 3 7 r ~ *benzaldehyde was determined from the analysis of the hyperfine structures of the EPR spectra obtained in the benzoic acid host. The obtained spin distribution in the triplet state is different from that of the benzaldehyde anion. It was shown that a UHF calculation explains the obtained spin distribution well.
Introduction The spectroscopic and photochemical properties of the lowest excited triplet (T,) states of carbonyl molecules have been the topics of much interest in recent years. We have been investigating the properties of the T1 states of both aliphatic and aromatic carbonyls by using optically detected magnetic resonance (ODMR) and conventional EPR techniques at liquid helium temperature in order to obtain a comprehensive understanding of their pr~perties.l-~ In this paper we discuss some of our recent observations concerning 3n7r* 2-indanone and 3 ~ 7 r *benzaldehyde. Information about the properties of the 3n7r* states of simple aliphatic carbonyls is needed in view of the theoretical interests in the zero field splittings (zfs) and decay properties of such molecule^.^^' Sharnoff and co-workers were the first to study 3 n ~ *aliphatic carbonyls by ODMR?9 but their studies were limited to polycrystalline or glass samples. We thought it desirable to make single-crystal ODMR studies of 3na* aliphatic carbonyls in order to obtain more detailed informations about their structures and decay properties. We found that the T1 state of 2-indanone can be used as a model of 3n7r* aliphatic carbonyl and therefore we made a single-crystal ODMR study at low magnetic fields. Here we present the result which indicates a large distortion from planarity in 3n7r* 2-indanone and discuss its decay properties in view of this distortion. As a representative of simple aromatic carbonyl we have chosen benzaldehyde and studied the properties of the T1 state in detail.1-4 The T1state of benzaldehyde is particularly interesting because of the proximity of the 3 n ~ * and 37r7r* states and mixing between these states makes the properties of the TI state remarkably dependent on e n ~ i r o n m e n t . ' ~ ~ ~Previous ~ J ~ J ~ ODMR investigations clarified the causes of the changes in zfs and the mechanisms of the decay processes in considerable detai1.113i4 Recently we found that the deuteration of the formyl hydrogen affects the zfs of 3ir7r*benzaldehyde greatly in some hosts. We discuss this observation in terms of the changes in the 3n7r* and 37r7r* state energies caused by deuteration. *Also a t the Department of Chemistry, State University of New York at Stony Brook, Stony Brook, N.Y. 11794.
0022-3654/79/2083-3350$01 .OO/O
In order to fully understand the properties of the T1 state of benzaldehyde we found it desirable to obtain information about the spin distribution. We have attempted to determine the spin distribution of 3 7 r ~ *benzaldehyde from the analysis of the hyperfine structures (hfs) observed in the single-crystal EPR spectra obtained in a benzoic acid host. Our results show that the spin distribution in 37r7r* benzaldehyde is rather different from that in the corresponding anion.
Experimental Section Durene, 1,6dichlorobenzene, and benzoic acid, used as host materials, were purified by extensive zone refining. Acetophenone, used as host, was purified by fractional distillation. 2-Indanone was purified by vacuum sublimation. Benzaldehyde, used as guest, was purified by distillation. Deuterated benzaldehydes obtained from Merck Sharp and Dohm were used without further purifications. All crystalline samples were obtained by growing crystals from melts following the standard Bridgman method. In Figure 1we give the molecular structures, the numbering, and the axis systems of 2-indanone and benzaldehyde used in this paper. Zero field ODMR experiments were made by using a setup very similar to that already described before1*except that a Spex 1704 spectrometer was used to analyze the phosphorescent emission. We followed the experimental procedures described in the literature to obtain zfs and decay parameters.13J4 ODMR experiments in the presence of a low magnetic field were made by placing the liquid helium cryostat containing a microwave helix between the pole faces of an electomagnet. Mixed crystals of durene containing small amounts of 2-indanone were mounted inside the helix so that 2-indanone is oriented in the desired orientation with respect to the direction of the applied field. The helix containing the crystal was rotated by turning the rigid coaxial cable attached to the helix at the top of the dewar. The sample crystal was mounted in the desired orientations with the aid of a polarizing microscope. The maximum errors in mounting are considered to be less than 5'. The ODMR signals were obtained by monitoring the phosphorescence at 404 nm and sweeping the microwave through the resonance frequencies a t about 0.7 GHz s-l. 0 1979 American Chemical Society
Triplet States of Aliphatic and Aromatic Carbonyls
1
X-Y
The Journal of Physical Chemistry, Vol. 83, No. 26, 1979 3351
1 (a)
z4y X
8
' V I
6&l: 5
2- indanon e
I
I (b) 4
benzaldehyde
Flgure 1. Molecular structures of 2-indanone and benzaldehyde, directions of the principal axes of the zero field tensors, and numbering of the atoms of benzaldehyde.
Signals were recorded after averaging 200 times. EPR measurements were made at 4.2 K by use of a spectrometer consisting of a JEOL ES-SCXA microwave unit with a cylindrical TEoll microwave cavity with an irradiation slit and a 100-kHz modulation and detection system. We have used a finger-tip dewar to do the liquid helium experiments. The details of the dewar and cavity assembly and the experimental procedures are similar to those reported previ0us1y.l~ A benzoic acid crystal containing a small amount of benzaldehyde was mounted on a wedge so that the applied field rotates within the molecular plane of benzaldehyde by rotating the sample holder. The mounting of the benzoic acid crystal was made by using the information given by a X-ray study.16
Results and Discussion Zero Field and Low Field ODMR Studies of 3 n ~ 2* Indanone. The phosphorescent spectrum of 2-indanone in durene host is shown in Figure 2a. This spectrum is entirely different from the well-known spectra of 3 n ~ * aromatic carbonyls which are characterized by progressions of 1700-cm-' carbonyl stretching frequencies. The 2indanone spectrum rather resembles that of cyclopentanone, indicating that the excitation is localized at the carbonyl moietya5 The phosphorescent spectrum has an extremely weak 0-0 band and is characterized by a long progression of -450-cm-l vibration. In the zero field ODMR experiments we observe two microwave transitions a t 2.83 and 2.15 GHz. By analogy to the case of cyclopentanone we assign the sublevel scheme with two fast decaying sublevels at the top and the bottom as shown in Figure 3. Both top and bottom sublevels are radiatively active. This decay characteristic is very different from those of 3 n ~ aromatic * carbonyls such as benzophenone in which only one ( 2 ) sublevel is radiatively active.17J8 The middle sublevel was assigned as the y sublevel with the y direction perpendicular to C=O as shown in Figure 1.6The results of the zero field ODMR experiments are summarized in Figure 3. In parts b and c of Figure 2 we show microwave-modulated phosphorescent spectra. Here we detect only changes in the phosphorescent intensities induced by rapidly sweeping the microwave through the resonance frequencies while scanning the monochromator. Since the middle sublevel is radiatively inactive, these spectra represent the sublevel spectra of the two fast decaying sublevels. Both spectra are similar under the resolution of the present experiment, indicating that both sublevels couple with the same singlet state. In the planar 3 n ~ (3A2) * carbonyl (C2")the three sublevels couple via spin-orbit coupling with l m * , 'nu*, and
Figure 2. Total and sublevel phosphorescent spectra of 2-indanone. (a) Phosphorescent spectrum of 2-indanone in durene. (b) L sublevel spectrum obtained by microwave frequency modulation at 2.8 GHz. (c) x sublevel spectrum obtained by microwave frequency modulation at 2.1 GHz. (d) High-resolution phosphorescent spectrum of 2-indanone neat crystal in the neighborhood of the 0-0 band. ki
ki'
PI
NI0
N
2154 GHz 13.0fi0.07
Figure 3. Sublevel and decay characteristics of 3 n ~ 2-lndanone: " k,, total decay rate constant; k,', relative radiative decay rate constant; PI, relative populating rate; NP; relative steady state populatlon. The sublevel scheme with the z sublevel top and the x sublevel bottom is also compatible with the experimental results and cannot be excluded.
states, respectively, in the following way:6J9120 (i) TJ3A1)- lna*('Al) (il) T#B2)* 'na*('B2) (iii) TJ3Bl)* l a ~ * ( ~ B ~ ) Since the oscillator strength for the So lag* transition is much larger than those for the So h a * and So lur* transitions only the z sublevel should be radiatively active. This was, however, not observed. 'UP*
-- -
3352
The Journal of Physical Chemistry, Vol. 83, No. 26, 7979
Hirota et al.
/
T
T
Z
i 270 /
T
T
[
Z
T
T
Z
&
t
2900
14.61
I
Ty
i
+TY;
3.:2Tx METHYL CYCLOHEXANE 3
nTC*
Tx
10.69
I
T 4j'
y Tx
3f5
Tx
, ~
,
O E ~ r = 2 0 0 c r i i 'dE,=365cm"
DICHLORO BENZNE
AETT>1500c6'
ACETOPHENONE B E N 2 0 6 A C I D 3KcJc*
Figure 5. Host dependence of the zero field splitting of 37r7r* benzaldehyde. The sublevel spacings are given In GHz. The decay rate constants of the z sublevels are given in units of s-'. Figure 4. Crystal structure of durene and the angular dependences of the transition frequencies at 320 G. Curves are calculated for 8 = ' 0 to 6 = 90' with I O o intervals. ( 0 )indicates the observed transition frequency. (- ---) indicates the transition frequency at zero field.
The observations that both z and x sublevels are radiatively active with similar sublevel spectra and that the phosphorescent spectrum shows a long progression of 450-cm-' vibration strongly suggest that 3n7r* 2-indanone is highly distorted from a planar structure in the crystalline system. It is, of cource, well known that 3na* aliphatic carbonyls such as formaldehyde have pyramidal structures in the gas phase.*% In order to obtain further information about the structure of 3n7r* 2-indanone, we have made low field ODMR experiments with durene as a host. In the durene crystal there are two molecules per unit cell which occupy different sites.24 The short axes of the durene molecules at both sites are nearly perpendicular to the ab cleavage plane as shown in Figure 4. We expect that 2-indanone replaces durene with its y axis nearly parallel to the short (M) axis of durene. When the magnetic field is applied perpendicular to the cleavage plane, the field direction is nearly parallel to the y direction of 2-indanone in both sites. The transition frequencies at both 2.8 and 2.1 GHz were shifted upward upon application of a magnetic field. This shows that the middle sublevel is the y ~ u b l e v e l . ~ We investigated the angular dependences of the transition frequencies by orienting the crystal so that the applied field rotated within the molecular plane of one type of durene as shown in Figure 4. The field direction rotated in the LM plane of durene at one site and in the NM plane a t the other site. In Figure 4 we plot angular dependences of the transition frequences predicted by the spin Hamiltonian 7f = gPII.9
+ DSz2+ E ( S X 2- S:)
(1)
with ID1 = 0.1303 and IEl = 0.0359 cm-I obtained from the zero field experiments for different 8. Here 8 is the angle between the direction of the z axis and the plane of field rotation as shown in Figure 4. 4 is the angle of the rotation of the applied field. At 4 = Oo the applied field is perpendicular to the ab cleavage plane. If 2-indanone is planar in the 3n7r* state and replaces durene with its molecular plane parallel to that of durene, the angular dependences of the signals should be close to the curves for 8 = Oo and 8 = 90°. In the presence of a magnetic field resonance lines become broadened and often showed structures possibly because of hyperfine interactions. Consequently, errors in the measurements are somewhat large. Nevertheless, the observed points fit to the curves for 40 and 50' approximately rather than those for 0 and 90° as expected for the planar 3n7r* state. Since
it is unlikely that 2-indanone substitutes durene with its molecular plane 40' away from that of durene, this is considered to be due to the out-of-plane distortion of the 3n7r* 2-indanone (C& When the 3n7r* state is distorted in this way, the x direction may no longer be perpendicular to the C=O direction and the x sublevel can become radiatively active as observed. Although d = 40° and d = 50° do not measure the angle of distortion directly, 3n7r* 2indanone is likely to be distorted very largely as in 3n7r* formaldehyde which is distorted by 36O from the planar structurea21 The zfs of 3n7r* 2-indanone and that of cyclopentanone are rather small in spite of the fact that the excitation is localized on the carbonyl moiety. Recent work on formaldehyde by Ramsay et al. also suggests a small value of DqZ5Hence the zfs of the 3n7r* aliphatic carbonyl may be considerably smaller than the value originally suggested for formaldehyde by Raynes.21 On the other hand, the ab initio calculation of the zfs by Langhoff et al. on planar formaldehyde gives D = 0.34 cm-1.6 It appears that a reliable calculation of the zfs based on a distorted structure of 3n7r* aliphatic carbonyl is needed. Zero Field ODMR and High Field EPR Studies of 3a7r* Benzaldehyde. (a)Deuterium Effect on the Z f s of 37r7r* Benzaldehyde. The zfs of 37r7r* benzaldehyde varies remarkably depending on the e n ~ i r o n m e n t In . ~ Figure ~~~~~ 5 we show how the sublevel energies of the TI state of benzaldehyde changes depending on the host. I t is seen that ET, - ETy(=Acz,) varies tremendously depending on the host, while ET - E T z (=At,,) remains nearly the same. I t is now we11 understood that this variation is primarily due to the changes in the contribution to zfs from the ~~'~ second-order effect of the spin-orbit ~ o u p 1 i n g . l ~The depressions of the sublevel energies of the 37r7r* state are given aS3,10,26,27
here 11n7r*u) and I3n7r*u') represent the u and u'th vibrational states of the lnr* and 3n7r*states, respectively. In the previous discussion of the zfs of aromatic carbonyls and conjugated e n o n e ~ ~ lthe ~ ~ above , ~ ' expressions were approximated by Gs/AEsT and GTIAE,, where AESTand A E T T are the energy separations between the electronic origin of the 37r7r* state and those of the lna* and 3n7* states, respectively. Gs and GT are the matrix elements for the spin-orbit couplings with the h * and 3na* states. This approximation is good when the contributions due
The Journal of Physical Chemistry, Vol. 83,No. 26, 1979 3353
Triplet States of Aliphatic and Aromatic Carbonyls
0
GHz
0
DC 8
.'
42b0
, I
I
I
41'50
4050
41'00
39(50 A
40'00
Flgure 7. Phosphorescent spectrum of benzaldehyde in benzoic acid host. (0)and (0')are the 0-0 band of the phosphorescent spectra of the two different species with different ZFS.
Flgure 6. Plot of Aczy vs. l / A € n - 1/A,EsT in acetophenone and 1,4-dichIorobenzene (DCB) host.
to the mixing between the zeroth vibrational states are dominant. If we further assume Gs N GT = G, Atzyis given approximately as Atzy = A E , ~ m T T
mST
Here At,? is the contribution to Atzy from spin-spin interaction in the pure 3 7 r ~ * state. The applicability of this equation has been tested for a series of substituted benzaldehydes and satisfactory results were obtained.'i3J0 Recently we found that deuteration of the formyl hydrogen changes At, of 3 7 r ~ *benzaldehyde largely in acetophenone and 1,4-&chlorobenzenehosts, though complete deuteration hardly affects Atzy in the same hosts. At, changes from 8.81 to 7.71 GHz and from 10.7 to 8.2 GHz in acetophenone and dichlorobenzene, respectively. Such a large deuterium effect is rare28and intriguing, but the present deuterium effect can be nicely explained on the basis of eq 3. According to the recent spectroscopic investigation by Kahlil, Hankin, and Goodman,29deuteration of the formyl hydrogen blue shifts the 3n7r*energy by 50 cm-l and hEm becomes considerably larger accordingly. On the other hand, complete deuteration blue shifts both %T* and 3 7 r ~ * energies by similar amounts, making AETT of both protonated and deuterated benzaldehydes similar. Using the AEm and a s Tvalues for benzaldehyde in acetophenone and dichlorobenzene previously determined3 and the above-mentioned shifts of the energies due to deuteration, we plot Aczy vs. l/Um- 1/mSTfor benzaldehyde-ha, -7-dl, and -d6 in Figure 6. The slopes of the lines give G = 10.4 cm-I for acetophenone host and 9.0 cm-l for dichlorobenzene host. These values are in good agreements with 9.0-cm-' value previously estimated for a series of G estimated in the two substituted ben~aldehydes.'~~J~ different hosts are slightly different indicating that the value of G depends on the host. The intercepts give At. due to spin-spin interaction only. The obtained Ae,;:fs very small. This is also consistent with the small value of Atzy found for benzaldehyde in benzoic acid host in which spin-orbit contribution is considered to be very small. hez also seems to depend on the host slightly. Spin Distribution in 3~7r*Benzaldehyde. The zfs and the decay rates of 3a7r*benzaldehyde in a benzoic acid host indicate that the Tl state is in a nearly pure 3 ~ 7 r * state. Thus this system is suited to study the spin distribution in 3 7 r ~ *benzaldehyde. The phosphorescent spectrum of benzaldehyde in a benzoic acid host shows two strong 0-0 bands due to different sites as shown in Figure
(6)
V
.10G.
V (bl
(ai
(CI
Flgure 8. Hyperfine structures of the EPR spectra of benzaldehyde observed at various orientations: (a) applled field nearly perpendicular to C4-H (b) and (c) applied field nearly perpendicular to C6-H; top; experimental; bottom; calculated.
7, but here we only discuss the EPR spectra of the species responsible for the 0-0 band at 25458 cm-l. Complete analysis of the EPR and ODMR results of the species a t different sites will be reported later. The zero field ODMR experiments made a t the 0 band of the phosphorescent spectrum shown in Figure 7 give the sublevel scheme shown in Figure 5. The observed angular dependences of the EPR signals agree quite well with those predicted from the zfs determined by the ODMR experiments, indicting that the EPR spectra discussed here are due to the species which give rise to the phosphorescent spectrum with the 0-0 band a t 25458 cm-'. The EPR spectra obtained at 4.2 K show resolved hfs when the applied field is in the molecular plane of benzaldehyde. Some representative spectra are shown in Figure 8 together with simulated ones. From the hfs observed a t various positions and the simulated spectra we can estimate the spin distributions in 37r7r*benzaldehyde. When the off-diagonal terms of the hyperfine tensor and the dipole interactions with nonnearest atoms are neglected the rth element of the hyperfine interaction due to the proton attached to the kth carbon is given by30 (4) (S'Arh= - g ~ , l ~ J I ~ J (+ Q kA d & In the following we assume that Qk and Akr are independent of k and use Q = -66.36 MHz, AI = 35.04 MHz, and A , = -40.36 MHz obtained for the triplet state of benzene.31 Here 1 and m refer to the directions parallel and perpendicular to the C-H direction in the molecular plane. When the applied field is making an angle cp with respect to the C-H direction, A is given by A = (A? cos2 cp Am2sin2 cp)1/2 (5) Using a Gaussian line shape for each hyperfine component, we have simulated the spectra. In simulating the
+
3354
The Journal of Physical Chemistry, Vol. 83, No. 26, 1979
TABLE I: Spin Distribution in Benzaldehyde _ _ _ _ _ _ _ I _ _ _ _ _ . ~
triplet (’nn*) I _ _ _ _
position
calcd
expt
1 2 3 4 5 6 7 8 a
-0.08 0.33
0.12 -0.03
anion radicala
___
(UHF)
expt
0.358 0.107 0.006 0.331 0.017 0.125 0.017 0.118
0.1977 0.0551 0.2730 0.0316 0.1432 0.3589
Taken from ref 32.
n* in
’nn*
calcdb (McLachlan) (INDO) calcd
0.0525 0.1904 0.0632 0.2425 0.0365 0.1433 0.3555 0.1155
Taken from ref 39.
0.006 0.099 0.001 0.112 0.002 0.098 0.537 0.146
Hirota et al.
benzaldehyde, we used the result of the IND039calculation for comparison. The INDO calculation predicts that only 32% of the R* electron distribution is in the phenyl ring. On the other hand, the present investigation shows that about 90% of the unpaired R electron is delocalized into the phenyl ring. This seems to show that the spin dis* 3 ~ states ~ * tributions in the P* orbitals of the 3 n ~ and are very different from each other. The differences in the R electron distributions in the ~ * seem to correlate with the observed 3 n ~ and * 3 ~ states energy shifts due to deuteration. Deuteration at position 4 produces a blue shift of the 37ra*energy by 36 cm-l, while deuteration at position 7 produces only a O--lO-cm-l blue shift.29 This correlates with the observation that p4 is large but p7 is very small in the 3 ~ state. ~ * On the other hand, deuteration at position 7 causes a blue shift of the 3 n ~ * energy by 50 cm-l. This correlate with a large R* electron density at position 7 in the 3 n ~ state. *
spectra we only consider the splittings due to the protons attached to carbons 2 , 4, 6, and 7, since p3 and p5 are expected to be small. When the applied field is nearly perpendicular to the C4-H direction, we observed a doublet with a splitting of 12.5 G. This gives p4 = 0.33 immediately. Acknowledgment. We thank Professor K. Nishimoto u4 is assumed to change according to eq 5 depending on of Osaka City University for helpful discussions regarding cp. From the simulated spectra in parts b and c of Figure UHF calculation. This research was partly supported by 8 we obtain a6 = 5.7 G. From the simulated spectra oba grant from the Ministry of Education of Japan. tained with the applied field perpendicular to C2-H and C7-H we estimate the values for u2 and u7. However, the References and Notes spectrum at this orientation does not show a well-resolved (1) T. H. Cheng and N. Hirota, Mol. Phys., 27, 281 (1974). spectrum and the estimates of u2 and u7 have large un(2) S.W. Mao and N. Hirota, Mol. Phys., 27, 309 (1974). (3) E. T. Harrigan and N. Hirota, Mol. Phys., 31, 663, 681 (1978). certainties. From these values we can estimate the ap(4) Y. Hirata and N. Hirota, Mol. Phys., in press. proximate values of p2, p6, and p7 by using eq 4. Since the (5) M. Baba and N. Hlrota, Chem. Phys. Lett., 64, 321 (1979). spin density calculation predicts large p1 and ps values, the (6) S.Langhoff and E. R. Davidson, J . Chem. Phys., 64, 4699 (1976). (7) J. L. Ginsburg and L. Goodman, Mol. Phys., 15, 441 (1968). effect of the dipole interactions with the nonnearest atoms (8) A. L. Shain, W. T. Chiang, and M. Sharnoff, Chem. Phys. Lett., 15, cannot be neglected for p1 and pB. We made this correction 206 (19721 by using the calculated spin densities for p1 and ps and (9) A. L.’Shain and M. Sharnoff, Chem. Phys. Lett., 15, 206 (1972); placing 112 of the spin densities 0.7 A above and below 16, 503 (1972). (10) H. Hayashi and S. Nagakura, Mol. Phys., 24, 801 (1972);27, 969 the molecular plane. The spin densities thus determined (19741. are given in Table I together with the spin densities for (11) M. Koyanagi and L. Goodman, J . Chem. Phys., 55, 2950 (1971); the benzaldehyde anion.32 The main observation about 57, 1809 (1972). (12) T. H. Chena and N. Hirota, J . Chem. Phys., 56, 5609 (19721. the spin distribution is summarized in the following: (1) (13)J. Schmidt,b. A. Antheunis, and J. H. van der Waals, Mol. Phys., p4 is very large, but p7 is very small in 3 ~ 7 r *benzaldehyde. 22, 1 (1971). A small value of p7 is in contrast to a very large p7 found (14) A. L. Kwiram, MTP Int. Rev. Phys. Chem., Ser. 1 , 4 (1972). (15) E. T. Harrigan and N. Hirota, J . Am. Chem. SOC.,97, 6647 (1975). in the anion. ( 2 ) In the triplet state p6 > p2, while p6 < p2 (16) G. A. Sim, J. M. Robertson, and T. H. Goodwin, Acta Crystallogr., in the anion. 8, 157 (1955). It is interesting that the spin distributions in the triple (17) W. S.Veeman and J. H. van der Waals, Chem. Phys. Lett., 7, 65 (1970). state and anion are rather different in benzaldehyde. This (18) M. Sharnoff and E. B. Iturbe, d. Chem. Phys., 62, 145 (1975). is different from the several known cases in which the spin (19) J. Sidman, J . Chem. Phys., 29, 644 (1958). distributions in the triplet states and anions are ~ i m i l a r . ~ ” ~ ~(20)J. C. Brand, J. Chem. SOC.,858 (1958). (21) W. T. Raynes, J . Chem. Phys., 44, 2755 (1966). In benzaldehyde the atomic orbital coefficients of the (22) L. E. Giddings and K. K. Innes, J . Mol. Spectrosc., 36,53 (1970). highest occupied molecular orbital (HOMO) and the lowest (23) G. W. Robinson and V. E. Digiogio, Can. J . Chem., 36, 31 (1958). unoccupied molecular orbital (LUMO) are different and (24) J. M. Robertson, Proc. R. Soc. London, Ser. A , 141, 594 (1933). ~ *and the anion are the spin distributions in the 3 ~ state (25) F. W. Birss, R. Y. Dong, and D. A. Ramsay, Chem. Phys. Lett., 18, 1 1 (1973). expected to be somewhat different even in the simple one (26)C. R. Jones, D.R. Kearns, and R. M. Wing, J . Chem. Phys., 58, electron excitation scheme. The differences in the orbital 1370 (1973). coefficients together with the effect of exchange polari(27)R. M. Hochstrasser, G. W. Scott, and A. H. Zewail, Mol. Phys., 36, 475 (1978). ~ a t i o probably n~~ produces the large difference observed. (28) The only other example of a large deuterium effect on the zfs to our In order to see whether or not the observed feature of the knowledge is in the case of l+benzoquinone. H. Veenvliet and D. spin distribution can be explained by a R electron MO Wiersma, J . Chem. Phys., 60, 704 (1974);Chem. Phys., 8,432 (1975). calculation, we have made an unrestricted Hartree-Fock (29) 0.S.Khalil, S. W. Hankin, and L. Goodman, Chem. Phys. Lett., 52, (UHF) calculation. We have used a procedure similar to 187 11977) that described by Amos and Snyder,37but the relevant (30) Cy A . Hutchison, J. N. Nicholas, and G. W. Scott, J . Chem. Phys., 49, 4235 (1968). matrix elements were evaluated by the method suggested (31) A. M. P. Goncalves and C. A. Hutchison, J. Chem. Phys., 49, 4235 by Nishimoto3* as described in a previous paper.35 The (1968). resonance integral P17was estimated as P17 = -0.51P + 1.7 (32) N. Steinberger and G. K. Fraenkel, J . Chem. Phys., 40, 723 (1964). (33) N. Hirota, C. A. Hutchison, and P. P. Palmer, J . Chem. Phys., 40, eV, where P is the bond order. The calculated spin den3717 (1964). sities are given in Table I. It is seen that the main feature (34) R. H. Clarke and C. A. Hutchison, J. Chem. Phys., 54, 2962 (1971). of the spin distribution is quite well explained by the (35) N. Hlrota, T. C. Wong, and E. T. Harrigan, and K. Nishimoto, Mol. Phys., 29, 903 (1975). present UHF calculation. (36) A. D. McLachlan, Mol. Phys., 5, 51 (1962). It is also interesting to compare the R electron distri(37) T. Amos and L. C. Snyder, J. Chem. Phys., 41, 1773 (1964). bution in the RR* and n r * triplet states. Since there are (38) K. Nishimoto, Theor. Chlm. Acta, 5, 207 (1966);7, 74 (1968). no experimental data on the spin distribution of 3 n ~ * (39) M. Koyanagi and L. Goodman, Chem. Phys. Lett., 21, 1 (1973). I
- . . I .
