580
Photochemical Behavior o€ Isocyanides
by
B.IC. Dunning, D. H. Shaw, and H. 0. Pritchard
Centre for Reseawh in Ezperimental Space Science, York Univeraity, Downitview, Ontario, Canada (Received May 6, 1970) Publication costro assisted by the National Research Council of Canada
Previous photochemical studies1v2have shown that at 2537 A, gaseous methyl isocyanide isomerizes to methyl cyanide without the involvement of radicals; further, that the limiting lowpressure quantum yield is close to 2, and that the isomerization is strongly quenched by inert diluents such as nitrogen or ethane, but readily sensitized by othelr molecules such as benzene, carbon dioxide, nitrous oxide, and possibly acetone. This note describes some exploratory quantum-yield studies for gaseous ethyl and phenyl isocyanides and for methyl, ethyl, and phenyl isocyanides in the liquid phase, a,ll experiments being conducted a t a wavelength of 2537 8.
Experimental Section Materials und Experimental Technique. Methyl, ethyl, and phenyl isocyanides were prepared by standard methods3 and quantum yields (a) for isomerization to cyanide a t 2537 were measured in the gas phase at 60" or in the liquid phase a t room temperature. The general procedures have been described already;'Z2 the systerns were mercury-free, and no thermal isomerization occurred under the conditions used in any of the experiments ; light intensities were determined by fernoxalate actinc~metry.~The minimum detectable quantum yield is governed by various experimental parameters such 81.1the light intensity, irradiation time, extinction coefficient, chromatograph sensitivity, and so on; in these experiments, the minimum detectable quantum yields are shown in column five of Table I, and yields 1t:ss than these are reported as nil in this note. Absorption Spectra. Methyl isocyanide exhibits a simple continuous absorption beginning',2 around 260 nrn with E = 0.3 1. mol-l cm-' at 2537 the absorption of ethyl isocyanide begicis at slightly longer wavelengths, and at 2537 %., E N 1.9 1. mol-' cm-l. Phenyl isocyanide, on the other hand, exhibits a typical aromatic absorption qectrum, beginning at 300 nm with maxima a 3 follow: A,, 274, 271, 264, 257, 231, 227, 223, and 219 nm; E : 54, 138, 146, 118, 4900, 4700,
A;
The Journal of I'hysica1 Chemistry, Vol. 76, No. 4, 1971
6300, and 4900 1. mol-' cm-I; at 2537 A, E = 100 1. mol-l cm-l. Liquid-Phase Photolyses at 95". All three isocyanides were photolyzed in hydrocarbon solution (pentane or hexane); also, MeNC and EtNC were photolyzed as neat liquids. In all cases, no isomerization could be detected, consistent with the gas-phase observation
Table I : Quantum Yields for Photoisomerization of Gaseous Isocyanides a t 2537 & . Molecule
T,OK
MeNC" EtNCb
356 333
PhNCb
333
* (10 mm) 1.4rt0.2
(1.5 i 0.5) x 10-1 (1.1 i 0 . 3 ) x 10-2
.p ( I
atm)
Detection limit
Nil Nil
zx 1x
(3.6 i 1 . 0 ) x 10-3
4X
10-3 10-3
From ref 1. This work; incident light intensity 2.0 X IOt4 quanta emm2see-'.
(see ref 1 and Table I) that collisional deactivation is a very efficient process for alkyl isocyanidee and moderately efficient for phenyl isocyanide. Further, it is known that for the alkyl isocyanides (as opposed to the phenyl compound), the isomerization is catalyzed by methyl and in all probability, photosensitized by acetone;' surprisingly cophotolysis of acetone with MeNC or EtNC either in hydrocarbon solution or as neat liquids yielded no isomerization, indicating highly efficient mutual deactivation processes in these systems. The only isocyanide which to our knowledge photoisomerizes in the liquid phase is o-biphenylyl isocyanide,e but in this case the reaction is the formation of another ring rather than the formation of the cyanide. Gas-Phase Photolyses at 60". Previous work1,2on NeNC has shown that at 83" arid 10 mm total pressure, Q, at 2537 is about 1.4, but that the isomerization is very strongly quenched by the addition of inert gas; for example, CP was reduced by a factor of 10
A
(1) D. H. Shaw and H. 0.Pritchard, J . Phys. Chem., 70, 1230 (1966). (2) D. H. Shaw, Ph.D. Thesis, Manchester, 1966. (3) I. Ugi, U. Fetzer, U. Eholzer, H. Knupfer, and K. Offermann, Angew. Chem., Int. Ed. Engl., 4, 472 (1965). (4) J. H. Baxendale and N. K. Bridge, J. Phys. Chem., 59, 783 (1955). (5) D. H. Shaw and H. 0. Pritohard, Can. J . Chem., 45,2749 (1967). (6) J. H. Boyer and J. DeJong, J . Amer. Chem. Soc., 91, 5929 (1969).
NOTES in the presence of 1011) mm of ethane2 and no reaction could be dtected at 1 atm total pressure.l Similar measurements at 2537 and 60" using 10 mm of EtNC gave 4, = 0.15, and the addition of 1 atm of ethane quenched the photolysis completely. Under the same conditions the quantum yield for isomerization of PhNC but in contrast, this at, 10 mm pressure is about isomerization i;. not strongly quenched by foreign upon addition of gas, and 4, falls only about 4 X just under 1 atm (6.50 mm) ethane. However, in common with MeNC, neither EtNC nor PhNC showed any evidence of the presence of free radicals in these photolyses.
Conelusions The gas-phasc results are summarized in Table I, and they show MeEC and EtNC to be similar in their general behavior, except that the former gives a somewhat larger quantum yield; however, since energy chains have be2n suspected in both photochemical' and chemically zctivat ed7systems containing MeNC, it is possible thai RtNC may more nearly represent the norm for the plrotochemical behavior of alkyl isocyanides. The temperalure coefficient for the MeKC photolybis observed in our earlier work' is insufficient to account for the quantum-yield difference between the two molecules. In the gas phase PhNC differs from alkyl isocyanides in two respec's: that the quantum yield for isomerization is rather low, arid that the isomerization is less efficiently quenched by added gases. The low quantum yield could be t h e comequence of a greater fluorcscence yield, but phenyl isocyanide i s a rather unstable substance, and fluorescence measurements in the gas phase may prove di%cult. The difference in quenching efficiency could be symptomatic of a difference in mechanism, perhaps that in the PhKC case the energy resides initially in the aromatic chromophore, whereas in the aliphatic rr,oleculen, it must initially enter via the isocyanide group ckrmmophore (it being understood that the language of chromophores corresponds to a very oversimplified drscrip tion of the electronic excitation process) ; if this were the case, an intramolecular conversion to a highly excited vibrational state of PhNC -vr-ould have to occur, and this state ~vouldrequire a number of collisions before its energy was reduced below that of the thermal threshold. The position of the thermal threshold is not known precisely, but, more so than with the alkyl isocyanides, thermal isomerization takes place verv readily indeed upon distillation, and it seems likely thai the threshold could lie a little below the value of 38 kcal/mol found for the alkyl comp o u n d ~ . " -Thua. ~ with a primary excitation of 112 Itcal/moR, the amount of vibrational energy that would have to be remwed muld be of the order of 70-SO kcal/moE; the removal of such a large excess of vibra-
581 tional energy would not t,ake place with unit collision efficiency. (7) D. H. Shaw, B. K. Dunning, and H. 0. Pritchard, Can. J . Chem., 47, 669 (1969). (8) F. W. Schneider and B. S. Rabinovitch, 1.Amer. Chem. Soc., 84, 4215 (1962). (9) K. M. Maloney and B. S. Rabinovitch, J . Phgs. Chem., 73, 1652 (1969).
Spectra and Cis-Trans Isomerism in Highly Bipolar Derivatives of Azobenzene
by G. Gabor* and E. Fischer Department of Chemistry, The Weizmann Institute of Science, Rehoroth, Israel (Received J u l y $7, 1970) Publication costs borne completelg by the Journal Physical Chemistry
of
The absorption spectra and the thermal and photochemical isomerization of aromatic azo compounds have been studied extensively. For azobenzene (AB) the maximum of the T-T* band is at 313 nm and is redshifted on substitution of one or two hydroxyl or methoxy1 group^.^-^ The trans isomer of AB is planar and therefore more stable than the cis isomer.' The activation energy of the cis -+ trans thermal conversion is about 23 kcal/mol for azobenzene and its except for those containing substituents which cause intramolecular changes, such as production of tautomeric species or internal hydrogen bonds4v6and thereby lower the energy of the activated intermediate. Even in these derivatives the thermal isomerization can be "frozen out" at] sufficiently low temperatures and the thermodynamic equilibrium which is 100% trans can be changed by irradiation. 3-6 Similar isomerization processes in stilbenes9 and in (1) E. Fischer, J. Amer. Chem. Xoc., 82, 3249 (1960), and earlier references quoted therein. (2) G. Zimmerman, L. Chow, and U. Paik, &id, 80, 3528 (1958). (3) (a) D. Gegiou, K. A. Musskat, and E. Fischer, ibid., 90, 3907 (1968): (b) J. Saltiel and E . D. Megarity, {bid., 91, 1265 (1969); 90,4759 (1 968). (4) (a\ G. Gabor. Ph.D. Thesis, Rehovoth, 1964: (b? G. Gabor, Y. Frei, D. Gegiou, M . Kaganowitch, and E. Fischer, Isr. J. Chem., 5 , 193 (1967). (5) G. Gabor and K . Bar-Eli, J. Phys. Chsm., 72, 153 (1958). (6) (a) G. Gabor, Y . Frei, and E. Fischer, ibid., 72, 3266 (1968); (b) G. Gabor and E. Fischer, ibid., 66, 2478 (1962). (7) (a) G. S. Hartley, J. Chem. Soe., 633 (1938); (b) R. J. Corrucini and E. C. Gilbert, J. Amer. Chem. Soc., 61, 2925 (1939). (8) (a) J. Halpern, 6. W. Brady, and C . A. Winkler, Can. J . Res., 28B, 140 (1950); (b) E. R. Talaty, Ph.D. Thesis, Ohio State University, 1957. (9) (a) Sh. Malkin, BuEl. Res. Counc. Iw,,A l l , 208 (1962): (b) G. S.Hammond, M. J. Turro, and P.-4. Leermakers, J. Phys. Chem., 66, 1144 (1962). The Journal oj' Physical Chemistry, Vol. 7 6 , N o . 4, 1971
