Alkynyl
Cations
2443
State”, P. 0. Lowdin. Ed., Academic Press, New York, N.Y.. 1966, p 253. (17) T. K. Ha, Chem. Phys. Lett., 30,379 (1975). (18) (a) E. M. Hoffman and T. E. Eames, J. Am. Chem. SOC., 91, 2169 (1969); (b) T. E. Earnes and E. M. Hoffman, ibid., 91, 5168 (1969); (c) E. M. Hoffman and T. B. Eames, ibid., 93, 3141 (1971); (d) A. H. Cohen
and E. M. Hoffman, inorg. Chem., 13, 1484 (1974); (e) C. Hambly and J. B. Raynor, J. Chem. SOC.,Dalton Trans., 604 (1974). (19) (a) H. Lemaire and A. Rassat, J. Chim. Phys., 61, 1580 (1964); (b) T. Kawamura, S. Matsunami, and T. Yonezawa, Bull. Chem. SOC.Jpn., 40, 11 11 (1987), see also P. J. Zandstra, J. Chem. Phys., 41, 3655 (1964). (20) V. Malatesta and K. U. Ingold, J. Am. Chem. Soc.. 95,6404 (1973).
Charge Distribution and Structure of Alkynyl Cations. An INDO Study C. U. Pittman, Jr.,* 0. Wilemon, J. E. Fojtasek, and L. D. Kispert‘ Department of Chemistry, The University of Alabama, University, Alabama 35486 (Received June 5, 1975) Publication costs assisted by the University of Alabama
Geometry-optimized INDO calculations were performed on a series of alkynyl cations, R1RzC+-C=CR3. The geometries, charge densities, ?r-bond orders, and *-orbital electron densities are discussed in terms of the relative contributions of the alkynyl and allenic resonance hybrids: RzC+-C=CR RzC=C=C+R. The allenic hybrid makes a larger contribution when R1, Rz, R3 = H than when R1, Rz,R3 = CH3. The allenic hybrid makes its largest contribution when R1, Rz = H and RS = phenyl. When R1 and/or R2 are fluorine, the contribution by the allenic hybrid is reduced. These results are compared to recent 13C NMR chemical shift observations.
-
Proton and I3C NMR spectroscopy has been used extensively to probe the structure and charge distribution in carbo ~ a t i o n s . l -Recently, ~ Olah et al. reported the I3C4and IH5 NMR spectra of a series of alkynyl cations, I, and, on
I
I1
the basis of the 13C NMR chemical shifts, concluded that the positive charge is extensively delocalized (Le., the mesomeric allenyl cations, 11, contribute extensively to the ions’ structure). In particular, both the a and y carbons were significantly deshielded where R1 = RP = R3 = CH3. Here, Olah predicted the contributions of I and I1 to be in about a 2:l ratio and the charge at C, should be about twice that a t Cy.* The allenic forms, 11, and vinyl cations have proved to be elusive to observe under stable conditions in Thus, it might be expected that the vinyl character of alkynyl cations would be stabilized less than their corresponding allylic cations. Indeed, the solvolysis rate of 3-chloropropyne was a factor of 100 less than 3-ch1oropropene.l However, Taft et al.9 found the relative stabilization energies of CHz=CH2CHz+ and CH=CHCHz+ vs. CH3+ were -2.35 and -2.40 eV, respectively, in the gas phase. Since we had already performed geometry-optimized theoretical studies, in the INDO approximation, on substituted allylic cations,1° extension of these studies to alkynyl cations was performed to probe their charge distributions.
Results Calculations were performed on the alkynyl cation series Ia-g. Geometry optimization was carried out by the “brute
Rl
>C+ -C=C-RJ R? Ri RL RJ I a H H H b CHj CH, CHI c H H Ph d H H F e H F F f F F F g CHI CHI F force” method as described p r e v i o ~ s l y . ~All ~ - ~bond ~ lengths and angles were fully optimized except for the C-H lengths and angles of the methyl groups and the phenyl ring of IC.The ring geometry in ICwas partially optimized. Calculations employed the CNINDO program, QCPE number 141. The optimized geometries are shown in Figure 1. In all cases the molecular plane was the xy plane. Thus, the pz orbitals were those perpendicular to the molecular plane. The x axis was defined along line formed by the linear C,-Cp-C, system. The favored C,C&, geometry was linear in all cases. An examination of the calculated bond lengths, charge densities, rz-bond orders, and T -and o-electron distributions in Ia-g provided a picture of both the electron distribution and (qualitatively) the magnitude of the contribution of hybrid 11. In every case, the C,-Co length was substantially greater than the Cp-Cy length. The C,-Cp lengths varied from 1.325 (IC)to 1.372 A (If) while the C r C , lengths varied from 1.220 (Ig) to 1.270 8, (IC).The C,-Co lengths decreased in the order If > Ig = Ib > Ie > Ia > Id > IC. Thus, progressive methyl and a-fluorine substitution seems to favor a larger contribution of hybrid I. These groups stabilize the cation and thereby reduce the contribution by I1 needed. The Journal of Physicai Chemistry, Vol. 79,
No. 22, 1975
2444
Pittman, Wilemon, Fojtasek, and Kispert ' 096 \
'.lo4
H-,
"
Id -
le
'\+,629FC -C --c
-.151 1.364
H+. 0 4 0
- 044
r.563 1.234
1'297
/ 0 70 1.370
,a,
-.a97 9"*,
F\
I I,
1,225
1.09"
1.297
H+.091
\ */
1.220
.09 1
Ph C--H
Figure 1. Optimized geometries and
\ -0
,,
A
1118A'
r043
charge densities of alkynyl cations la-g.
The strongest contribution by allenic hybrid I1 is found in ICwhere a phenyl ring is attached to C,. For resonance stabilization of the phenyl ring to be manifest, allenic participation is required. The strong allenic contribution in IC +CHL-CEC
* I , ,
CH,=C=-C+
-0-
while C, substitution increases allenic character. This is clearly evident on examining the a,-bond orders. Thus, the C,Cp a,-bond orders decrease in the order Id (0.732) > Ie (0.604) > If (0.522). Similarly the CpC, a,- bond orders increase in the same order, Id (0.658) < Ie (0.752) < If (0.801). Summarizing, hybrid b of Id is a larger contributor than is hybrid c of If.
-
+
IC
C H 2 = C = CG should lead to an abnormally high C,Cp bond order and this is found. The C,Cp *,-bond order (Table I) is 0.809, which is substantially greater than those for the rest of the ions. Furthermore, the phenyl ring is strongly quinoid distorted. The ring Cortho-Cmeta bond lengths are shorter than the others and the Cortho-Cmeta a,-bond orders (0.711) are higher than those for the other ring bonds. Finally, the charge densities and the CrCipso a,-bond order (0.540) agree with substantial contribution by allenic hybrids. The bond lengths and a-bond orders show that the allenic contribution in Ia is greater than in its methyl analog Ib. For example, the C,Cp a,-bond order decreases while the CSC, a,-bond order increases going from Ia to Ib. Fluorine substitution at C, also decreases the allenic contribution The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
CH,+%C-F a
CH,=C=C-F
+
CH,==C==C=F+
b Id
C
b
a
F
\
C=C=C-F
+
F' C
If
The donation of fluorine .rr,-electron density occurs to an appreciable extent. Thus, in Ie and If the C,F a,- bond or-
2445
Alkynyl Cations
TABLE 111: Electron Density ( q ) in Selected Orbitals of Ions Ia-g
TABLE I: a-Bond Orders in Alkynyl Cations
Ia
0.735 0.958 Ib 0.569 0.766 Y 0.937 Ica z 0.809 0.564 0.542 Y 0.924 Id z 0.732 0,658 0.460 Y 0.934 0.209 Ie z 0,604 0.752 0.513 0.398 Y 0.93 8 0.242 If z 0.522 0.801 0.467 0.367 Y 0.93 1 0.264 Ig z 0.528 0.780 0.354 Y 0.951 0.201 a T h e phenyl ring exhibited a quinoid distortion (see Figure 1) and its n-bond orders alternated: C i p s o - C n r t h o (0.5401, C n r t h o Cmeta
z Y z
(0.7111,
0.675
Corthii-Cpara
(0.632).
TABLE 11: Carbon-Fluorine a-Bond Polarization in Alkynyl Cations Id-g Values of qpE Cation
C,
F,
CY
FY
Id Ie If Ig
0.769 0.755
1.436 1.444
0.776 0.776 0.775 0.768
1.397 1.389 1.385 1.388
ders are large (0.513 and 0.467 respectively). The C,F a,bond orders were 0.460, 0.398, and 0.367 in Id, Ie, and If, respectively. Also, a weaker F,,-C, ay overlap exists (see Table I). This donation of electron density does not lead to a buildup of positive charge on the fluorines or a decreased total positive charge density at C, or C, in the fluorinated ions Id-If relative to Ia. Instead, this back-a-donation of electron density is more than counterbalanced by the strong polarization of the C-F u bonds. Each C-F u bond is strongly polarized with the values of qp, for carbon about 0.77 and for fluorine about 1.39. (See Table 11.)Therefore, the positive charge at C, in Id-g and at C, in Ie and If is greater than that at C, or C, in Ia. The distribution of electron density in the a orbitals of cations la-g are listed in Table 111. The electron density ( q ) is low in the p, orbitals of C, and C,, the carbons on which the major fraction of positive charge is found. The “nonconjugating” carbon is not substantially charged and has an electron-rich p, orbital (in every case qpz > 1).Further examination of Table I11 shows the phenyl ring donates a density from the ortho and para positions, as expected. The fluorine atoms donate electron density from pz orbitals but not pr orbitals. Conclusions These calculations indicate more localization of charge a t C, than at C, (Le., less contribution by hybrid 11) in the tertiary alkynyl cation Ib. This agrees with the 13C chemical shifts and their interpretation by Olah et al.4 c, in Ib was found 269.0 ppm downfield of TMS compared to a 219.1-ppm value for C,. The values of A6 for these carbons (value going from the precursor alcohol to the cation) were
OrbiIon tal Ia
C,
CB
c,
FY
Fa
1.063 0.508 0.944 1.080 Ib 0.546 1.116 0.670 0.994 1.062 PY Ica pe 0.665 1.050 0.611 1.053 1.005 PY Id pp 0.497 1.099 0.557 1.847 0.996 1.074 1.953 PY Ie pe 0.526 1.131 0.650 1.878 1.815 1.073 0.965 1.943 PY If p1 0.567 1.153 0.701 1.892 1.844 PY 1.126 0.899 1.936 Ig pz 0.516 1.136 0.700 1.899 PY 1.017 1.057 1.957 a The values of q p , for the phenyl ring were as follows: C,,,,,, 1.060;C n r l h o , 0.886; C m p t a , 1.002; C p a r a , 0.839. pz
0.429
PY pE
204.0 and 141.4 ppm. In contrast, the A6 for Co was only 26 ppm. These values are consistant with our calculated charge distributions, a-bond orders, and orbital electron densities. Olah’s conclusion that the relative contribution of alkynyl (I) and allenic (11) hybrids in Ib was 2:14 is a reasonable one based on our calculations, but the amount of positive charge at C, (+0.347) is not twice that at C, (+0.196) as s u g g e ~ t e d . ~ An interesting comparison may be made between alkynyl cations Id and If and the fluorinated allylic cations I11 and IV.l0 The calculated CoC, lengths in Id and If (1.243 and H4-0 11’1
I
F L 112
t 0 “i 1H
111
F
F-(1 i o 4 IV
1.225 A, respectively) are significantly shorter than those in
I11 and IV. Furthermore the C,Cp lengths in Id and If
(1.336 and 1.372 A) are slightly shorter than those in I11 and IV. Therefore, within the INDO framework, the C&, bonds in Id and If appear to be largely triple bonded. The C,Cp bonds of Id and If (formally sp2-sp) appear slightly shorter than the C,Cp bonds of I11 and IV (formally sp2sp2). The same patterns of back-a,-donation from fluorine to carbon and u polarization from carbon to fluorine were manifest in both series. These calculations suggest that I3C NMR studies of fluorinated alkynyl cations of Id and If and allylic cations such as I11 and IV would be very interesting. The 13C chemical shifts could be compared with the calculated charge densities, a-orbital electron densities, hybridizations, and the substitution pattern. For example, in Id and If and in I11 large differences in charge densities exist a t C, and C,. Their 13Cchemical shifts should reflect these differences. Acknowledgment. The University of Alabama Computer Center is thanked for generous amounts of free computer time. The Journal of Physical Chemistry, Vol, 79,No.22, 1975
2446
Hisao Murai and Kinichi Obi
References and Notes (1)C. U. Pittman, Jr., and S. P. McManus in "Reactive Intermediates in Organic Chemistry", S. P. McManus, Ed., Academic Press, New York, N.Y., 1973. (2)G. A. Olah and J. A. Olah in "Carbonium ions", VoI. 11, G. Olah and P. v. R. Schleyer, Ed., Wiley-lnterscience, New York, N.Y., 1970, pp 715-
782. (3) G. A. Olah and C. U.Pittman, Jr., Adv. Phys. Org. Chem., 4 (1966). (4)G. A. Olah, R . J. Spear, P. W. Westerman, and J. M. Denis, J. Am. Chem. SOC.,96,5855 (1974). (5) C. U. Pittman, Jr., and G. A. Olah, J. Am. Chem. SOC.,67, 5632 (1965). (6)P. J. Stang, Prog. Phys. Org. Chem., 10, 276 (1973). (7)P. J. Stang and T. E. Dueber, J. Am. Chem. SOC.,95,2683 (1973). (8)2 . Rappoport, T. Bassler, and M. Hanack, J. Am. Chem. SOC..92,4985 (1970).
(9)R. W. Taft. R. H. Martin, and F. W. Lampe, J. Am. Chem. SOC., 87, 2490 (1965). (IO)L. D. Kispert, C. U. Pittman, Jr., D. L. Allison, T. B. Patterson, Jr.. C. W. Gilbert, Jr., C. F. Hains, and J. Prather, J. Am. Chem. SOC.,94, 5979 (1972). (11)L. D. Kispert. E. Engelman, C. Dyas, and C. U. Pittman, Jr.. J. Am. Chem. Soc., 93,6948 (1971). (12)C. U. Pittman, Jr., C. Dyas, C. Engelman, and L. D. Kispert, J. Chem. SOC.,Faraday Trans. 2,68, 345 (1972). (13)C.U. Pittman, Jr.. T. B. Patterson, Jr., and L. D. KisDert. J. Org. Chem., 38, 471 (1973). (14)c. U. Pittman, Jr. A. Kress, T. B. Patterson, P. Walton, and L. D. Kispert, J. Org. Chem., 39, 373 (1974). (15)C. U. Pittman, Jr.. A. Kress, and L. D. Kispert, J. Org. Chem., 39, 378 1197A\ ,.-. . I .
(16)C. U. Pittman, Jr., L. D. Kispert, and T. B. Patterson, Jr., J. Phys. Chem., 77, 494 (1973).
Photochemistry of Higher Excited Triplet States of Benzaldehyde, Acetophenone, and Benzophenone at 77 K Hisao Mural and Klnichi Obi* Department of Chemistry, Tokyo Institute of Technology,Ohokayama, Meguro-ku, Tokyo, Japan (Received February 4, 1975; Revised Manuscript Received August 1, 1975)
The photochemistry of benzaldehyde, acetophenone, and benzophenone has been investigated a t 77 K under high-intensity irradiation. The reactions take place through the higher excited triplet state formed by a biphotonic process. Benzaldehyde in the higher excited triplet state decomposes to benzoyl and atomic hydrogen or is converted into a ketyl radical. Onthe other hand, acetophenone and benzophenone undergo only ketyl radical formation. The lowest triplet state of these carbonyl compounds shows no hydrogen abstraction reaction at 77 K.
Introduction The hydrogen abstraction reaction of the n,r* triplet state of benzophenone1 has been extensively studied by a number of workers. It has been established that the following reaction occurs at room temperature Ph&O*(TI)
+ RH
--t
PhzCOH
+R
where RH is the solvent molecule. At low temperature, only a few studies have been made on the photochemical reactions of benzophenone. Sharp and coworkers2 have observed the EPR signal of the ketyl radical at 123 K. Kuwata and Hirota have observed a broad singlet EPR spectrum in pure crystals and in ethanol and EPA solutions of benzophenone at 77 K by irradiating the samples with ultraviolet light for several hours.3 Farmer, Gardner, and McDowell have suggested that diphenylketyl radicals are observed at 77 K by EPR mea~urernent.~ On the other hand, Godfrey, Hipern, and Porter5 have proposed that the ketyl radical was formed in the flash photolysis of benzophenone in isopentane solution at temperatures higher than but not below -100 K. The formation of the ketyl radical a t 77 K is, therefore, questionable. In general, aromatic molecules in rigid media can undergo the photosensitization reaction by the higher excited triplet state through a biphotonic process. Detailed studies of the energy-transfer process from the higher triplet state The Journal of Physical Chemistry, Voi. 79, No. 22, 1975
have been carried out for naphthalene, naphthalene derivatives, and aromatic Since these molecules have fairly long triplet lifetimes in rigid media, it is easy to make the triplet concentration high by light irradiation. On the other hand, since the lifetimes of the triplet states of aromatic carbonyl compounds are very short at 77 K compared with other aromatic molecules, no study has been reported on the biphotonic process of these compounds. In order to observe the biphotonic process of these molecules, it is necessary to use a high-intensity lamp as a light source. In this work, the biphotonic processes of benzaldehyde, acetophenone, and benzophenone have been investigated at 77 K using an extra-high-intensity mercury lamp. Experimental Section The EPR spectrometer used in this experiment was a conventional X-band type (JEOL JES-3BS-X) operated with 100-kHz modulation. In order to cool the sample, a quartz dewar or a variable-temperature dewar was inserted into a TEol1 cylindrical cavity. A 3- or 4-mm 0.d. quartz tube was used for the EPR measurements. Mn2+ doped in MgO powder was used as a standard for the intensity and the hyperfine splitting of the spectra. The optical absorption spectra were measured a t 77 K using a Hitachi EPS-3T spectrometer. In order to prevent the rise of sample temperature, the absorption cell was im-
