J. Phys. Chem. 1984,88, 1317-1320 other members of the n-GaAsl-,P, series. We certainly expect that the long-term stability of the n-GaAsl-,P, series will be comparable to the low etch rate (- 1 pm/year) of the n-GaAs system.22 The investigation of the properties of the n-GaAsl-,P, series can also be useful in understanding the reported surface chemistry of n-GaAs. Treatment of n-GaAs with RuC13 has been reported to be effective in passivating surface states in KOH/SeZ- solut i o n ~ . ’ ~This * ’ ~ is also reported to yield decreased recombination velocity and increased luminescence lifetimes at the n-GaAs/ vacuum interface.50 We observe substantial improvement in photocurrent-voltage properties of the n-GaAsl,P, series for x < 1.0 after exposure to RuC13. An example of the difference in photocurrent-voltage properties is depicted in Figure 8 for several compositions of n-GaAsl,P,. As for n-GaAs, the main effect of RuC13 is to improve the fill factor in KOH/Se2- solutions. The values of V, and I , are unaffected by this procedure. We observe similar effects for every member of the series n-GaAsl,P, except for n-GaP itself. We can thus associate the effect of RuC13 treatment to changes in interfacial kinetics arising from coordination with arsenic or arsenic oxide sites on the electrode surface. This is consistent with theoretical treatments of the electron distribution at a GaAs surface, which place increased electron density on the As sites.51 Studies of the bonding of RuC1, to As in the dilute matrix provided by n-GaAso,05Po,95 are being performed in order to establish a more detailed picture of the interactions in this system. Finally, we note that the values of V , for the semiconductor/liquid junctions exceed that for Au Schottky barriers for every member of the n-GaAs,,P, series. It has been suggested that the Fermi level of n-GaAs is so severely pinned by surface states ~
~~
~
Nelson, R. J.; Willarns, J. S.; Leamy, H. J.; Miller, B.; Parkinson, B. A,; Heller, A. Appl. Phys. Lett. 1980, 36, 76. (50)
(51) Goddard, W. A., III; Barton, J. J.; Redondo, A.; McGill, T. C. J . Vuc. Technol. Sci. 1978, 15, 1274.
1317
that formation of any barrier, whether a metal overlayer or a liquid junction, would not alter the amount of band bending in the resulting d e ~ i c e . ~ ~InJ *direct comparisons of Schottky barriers and liquid junctions formed with identical semiconductor material, we consistently find that for n-Si in MeOH,18 n-GaAsI9 and n-GaAsl_,P, in acetonitrile,” and now for n-GaAsl,P, in aqueous KOH/Se2-, the values of liquid junction V, exceed those demonstrated for simple Schottky systems under identical illumination conditions. In some cases, such as with n-Gap, this difference can be as large as 0.5 V.” There is increasing evidence that the interfacial chemistry of the semiconductor/contact j ~ n c t i o n ’ ” ’ ~ ~ ~ ~ ~ ~ ~ is more responsible for the values of &, and V, than is the intrinsic distribution of surface states in the semiconductor itself. Simply replacing the metal overlayer (even a so-called noble metal) in a Schottky barrier by a liquid junction may result in a sufficiently large chemical change at the interface to invalidate predictions of V, for liquid systems based upon measured values of Schottky contacts. Design of systems that take advantage of these differences between metal and liquid interfaces is the key to formulating efficient semiconductor/liquid junction devices. Acknowledgment. We gratefully acknowledge C.L.R. Lewis of Varian Associates, Palo Alto, CA, and R. Farraro and L. Stinson of Hewlett-Packard, Inc., Palo Alto, for growth and characterization of some of the n-GaAsl,Px samples used in this study. We also thank G. Cogan, J. Gibbons, L. Christel, and G. Moddel of SERA Solar Corp., Santa Clara, CA, for many helpful discussions and for the use of metal evaporation and spectral response facilities. This work was supported by the Stanford Center for Materials Research, funded by the National Science Foundation, and by the donors of the Petroleum Research Fund, administered by the American Chemical Society. Registry No. Ruthenium(3+), 22541-88-4; gold, 7440-57-5. (52) Fan, F-R F.; Hope, G. A.; Bard, A. J. J. Electrochem. SOC.1982, 129, 1647.
Geometry of the *CH20R Radical in X-Irradiated Crystals of Methyl 0-D-Galactopyranoside: An ESR/ENDOR Study William A. Bernhard,* Tex L. Homing, and Kermit R. Mercer Department of Radiation Biology and Biophysics, The University of Rochester School of Medicine and Dentistry, Rochester, New York 14642 (Received: June 21, 1983)
A CHzOR radical is trapped in single crystals of methyl P-D-galactopyranosideX-irradiatedat
-
12 K. ENDOR measurements at -6 K were used to determine the two a-hydrogen and one y-hydrogen hyperfine coupling tensors. The a-hydrogen tensors, determined to a high degree of accuracy, were used to calculate the geometry about the free-radical center. The .CH20R radical is slightly bentwith 8, = 2.3.h l.Oo, where 6 is the angle between the nodal plane of the lone electron orbital and the three a-bonds (0-C, H-C, H’-C). The three angles H-C-H’, 0-C-H, and 0-C-H’ are 125.7 f 0.7’, 116.9 f 0.8’, and 117.2 f 0.8O, respectively. With a two-center dipole approximation the “H-C bond lengths are found to be ca. 1.00 8,that is, about 0.10 8, shorter than the H-C bonds in the nonradical. The unpaired electron densities are p c 1 0.83 and po 5 0 . 1 7 .
Introduction A question of long-standing interest is to what degree, if any, oxygen substituents induce nonplanarity (bending) about the free-radical center of oxyalky radicals. Simple alkyl radicals such as .CH3 and .CH20H have played a pivotal role in answering this question. Measurements of isotopic 13Chyperfine splitting a(13C) in a series of small substituted alkyl radicals led Fessenden and co-~orkers’-~ to conclude that the -CH3radical oscillates about (1) R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 43,2704 (1965).
0022-3654/84/2088-1317$01.50/0
an equilibrium conformation that is essentially planar’pz but that the C H 2 0 H and .CH20- radicals are bent slightly in their equilibrium conformation^.^ The angle of bending, 8, between each of the three a-bonds and the plane normal to the lone electron orbital (LEO) was estimated from a(13C) measurements to be -4O. They also noted that changes in hyperfine splitting (hfs) of the a hydrogen a(*H) might be the result of bending, as 0 increases a(*H) becomes less negative, and then turns positive.1,2 (2) R. W. Fessenden, J. Phys. Chem., 71, 74 (1967) (3) G. P. Laroff and R. W. Fessenden, J. Chem. Phys., 57, 5614 (1972).
0 1984 American Chemical Society
Bernhard et al.
1318 The Journal of Physical Chemistry, Vol. 88, No. 7, 1984
Measurements of a(*H), a(@H),and a(13C) on a series of cyclic and acyclic oxygen-substituted alkyl radicals provides convincing evidence that increased bending is indeed reflected in a("H) by shifting it toward positive values!,5 Dobbs et al.4 calculated, using INDO to reproduce the experimental values of a(13C) and a("H), that for .CHzOH the angle 0 is -4O. It has also been suggested6 that -CHzOCH3 and . C H 2 0 C H z 0 C H 3 are more bent than .CH20H. The general conclusion from these studies was that oxygen-containing substituents induce bending due to a M+ effect."6 That is, through conjugation the electron density at the radical center is increased, which in turn causes bending due to the increased Coulombic repulsion between the nonbonding 2p electrons and the a-bond electrons. At higher temperatures, the rapid interchange of the a hydrogens renders the a("H)'s equivalent, for example: C H 2 0 H yields ua = 48.7 MHz at 26 OC and .CHzOCH3 yields "a = 48.27 MHz at -40 OCe6At lower temperatures, the slowed interchange of "H's results in inequivalent a(aH)'s; for .CHzOH "a1 = 5 1.88 MHz, "a2 = 49.92 M H z at -125 OC and for .CHz0CH3 "a1 = 51.24 MHz, "a2 = 47.04 MHz at -100 OC. For .CH20H Krusic et al. have measured the temperature dependence of a("H) and a(0H). Their results, coupled with INDO calculations led to the conclusion that the .CH20H radical is essentially planar and that the "H trans to the 0-H bond gives rise to the larger value of u("H).~ The prediction that the trans "H gives the larger a("H) is supported by the results reported herein. Trapping of the .CHZOR radical in methyl P-D-galaCtOpyranoside (PMeGal) provides the opportunity to study the geometry of an oxy-substituted radical in detail. It is trapped in single crystals X-irradiated at 77 K or below and is formed by the net loss of hydrogen from the methyl group. The structure of PMeGal, along with the numbering system employed in the X-ray diffraction study, is as follows:
&MeGa I
CH,OR
A combination of the known crystal structure, lack of heavy-atom reorientation upon radical formation, and a very accurate set of "H hyperfine coupling tensors has made it possible to determine the degree of bending, the inter a-bond angles, and the torsion angles about 01-C7 and to estimate the "H-C bond lengths. Materials and Methods Polycrystalline methyl P-D-galactopyranoside (PMeGal) was purchased from Sigma Chemical Co. and single crystals were grown from H 2 0 and D 2 0 by cooling a saturated solution from 50 to 35 "C or by evaporation. The crystals were verified to have the r e p ~ r t e d space ~ , ~ group of P212121and unit cell dimensions of a = 7.778, b = 8.533, and c = 13.132 A by use of a Buerger precession camera. The precession camera was also used to align the crystals for data collection in the three crystallographic planes bc, ca, and ab as well as a fourth skew plane c'a'. Accurate alignment of the skew plane was critical to the overall accuracy of the final tensors. The transformation for the skew plane was 0.0092 0.9999 0.6586 0.7524 7524 0.6587 0.0000
0.0080
(4) A. J. Dobbs, B. C. Gilbert, and R. 0. C. Norman, J . Chem. Soc. A, 124 (1971). ( 5 ) A. J. Dobbs, B. C. Gilbert, and R. 0. C. Norman, J. Chem. Soc. Perkin 2, 786 (1972). (6) C . Gaze and B. C. Gilbert, J . Chem. Soc., Perkin 2, 116 (1977). (7) P. J. Krusic, P. Meahin, and J. P. Jesson, J . Phys. Chem., 75, 3438 (1971). (8) B. Sheldrick, Acta Crystallogr., Sect. B, 33, 3003 (1977). (9) S. Tagagi and G. A. Jeffrey, Acta Crystallogr., Sect. B, 33, 3576 (1977).
b AXIS
9=2,0036
1
n
:I \,
1
,
2 . 0 mT
w
Figure 1. First-derivative X-band ESR spectra of PMeGa! X-irradiated at -12 K and observed at 6 K. Stick diagrams mark the hyperfine splitting pattern due to the C H 2 0 R radical: top, -Ho\lb;bottom, -Holla.
where c' is 0.67' from c and a' is 48.80' from a and nearly in the ab plane. ESR and ENDOR data were obtained with an experimental configuration built around a Varian E- 12 that has been described previously.1° Two changes in the system have been made. One involves frequency modulation of the ENDOR rf source (Wavetek Signal Generator) instead of the previous field modulation technique. An FM modulation frequency of 100 kHz was used with a deviation of between 50 and 100 kHz. The other change was that the entire data collection, storage, and plotting were controlled by two Commodore microcomputers, a 4016 and a 2001. Analysis of the ESR and ENDOR data was performed with methods and formalism described previously.' It is important to note that the tensors and the variancmvariance elements were determined from ten data sets, two sets from the two magnetically distinct sites in each of the three cfystallographic planes bc, ca, and ab and four sets from the skew plane c'a'. In the skew plane there are four magnetically distinguishable sites. Inclusion of the skew plane not only ensures a physically unique solution but also, because of the high precisiog of crystal alignment, considerably enhances the accuracy of the tensor elements. Care was also taken to verify that the two a hydrogen hyperfine tensors were properly correlated to the same crystallographic site. This was done by testing all combinations to find the one that most accurately recreated the ESR spectra. There was no ambiguity in making this correlation. For the y-hydrogen tensor and g tensors, however, it was not possible to follow the data in the skew plane and these tensors are ambiguous as to the sign of the off-diagonal elements. On the other hand, the off-diagonal elements are quite small and given the other sources of error in determining g and YA this had little impact on the analysis.
'
Results and Analysis The ESR spectra shown in Figure 1 were taken at -6 K after X-irradiation of PMeGal at 12 K. These data were collected from DzO grown crystals. Along the b axis the two nearly equal "H hfc's give a 1:2:1 triplet while along the a axis and additional coupling due to the y hydrogen becomes apparent. These three hyperfine interactions, @lA,a2A, and ?A, were measured at -6 K in four different planes, as shown in Figure 2. The solid lines in Figure 2 were generated from the tensors given Table I. There
-
(10) D. M. Close, G. W. Fouse, and W. A. Bernhard, J . Chem. Phys., 66, 1534 (1977). (11) G . W. Fouse, Jr., and W. A. Bernhard, J . Mag, Reson,, 32, 191 (1 s i 8 j.
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1319
. C H 2 0 R Geometry in PMeGal
TABLE I: Principal Values, Principal Axes, and Variance-Covariance of the Hyperfine Coupling and g Tensors for the CH,OR Radical label
principal value: MHz
1
principal axisb m
n
(1,1)
variance-covariance matrixCx lo4 (13 (2,2) (23)
(13
(3,3)
87.3 (3) 51.6 (3) a'Amin 20.3 (3) "'4so 53.1 (2)
0.1901 0.9621 0.1953
0.8380 -0.2627 0.4782
0.5114 0.0728 -0.8562
0.325 0.037 0.499
-0.078 0.120 -0,099
-0.008 0.058 -0.058
0.033 0.413 0.058
0.025 0.097 -0.096
0.383 0.418 0,019
87.1 (2)
-0.2166 0.9589 0.1830
-0.8953 -0.2699 0.3544
0.3893 -0.0871 0.9170
0.216 0.032 0.506
0.052 -0.090 0.095
0.001 0.076 -0.064
0.022 0.280 0.067
-0.021 -0.126 0.007
0.049 0.446 0.016
"'A,,,
QIAillt
Q2Amax OIAint a'Amin
48.0 (3)
20.3 (2) 51.8 (2) A ', 15.5 (2) 0.981 -0.193 -0.027 0.076 0.374 0.065 1.898 0.007 2.355 'Aint 1.6 (9) YA,in 1.3 (9) 'Ais0 6.1 (4) gmax 2.0042 (1) 0.114 -0.130 -0.985 11.4 -13.9 -0.7 73.2 18.7 5.9 Bint 2.0035 (2) 0.07 0.99 -0.12 317.4 247.0 10.5 202.2 1.6 11.7 ginin 2.0030 (2) -0.99 0.05 -0.13 197.3 210.4 -86.0 228.0 95.2 55.7 &so 2.0035 (2) Direction cosines 1, m, and n are with respect to the a, b, c crystallographic axes. Six a Errors in the last digit are given in parentheses. .&que elements of the symmetric 3 X 3 matrices are given for each principal axis. The variance for a direction cosine is the corresponding digiving a standard deviation of (0.413 X = 0.643 X 10.'. agonal element. For example, for m of QIAint the variance is 0.413 X "'Aiso
See ref 11 for details.
ca
,601
I
ab
bc
I
I
c'a'
ANGLES 8 ;
TORSION ANGLES
P
2.3 f 1.00
Cl-01-C7-H?-114.2 f0.7. C1-01-C7-H?'= 13.310.7' HI-C1-01-C?= 45.1.
H7-C7-H7'=125.42 0.7' 01-C7-H7'=116 9 I 0 . 6 ' oi-c7-n7:l1-.21 0 . 8 ~
N
e e o o /
10
r *,r--+--7b
C a' CRYSTAL O R I E N T A T I O N ( 3 0 j D I V ) Figure 2. Angular variation of the ENDOR transitions associated with the .CH20R radical at -6 K. The data marked by X, 0, and + were used to calculate the tensors nlA, u2A,and YA, respectively. The solid lines were calculated from the tensors reported in Table I. The c'u'plane is skew to the unit cell axes with ~'0.67' from c and ~'48.80' from a and nearly in the ab plane. C
a
were no noteable differences in the . C H 2 0 R spectral features between H 2 0 and D 2 0 grown crystals. As can be seen from the goodness of fit in Figure 2 and the calculated standard errors in Table I, the *'A and a2Atensors have been determined to a high degree of accuracy. On the other hand, 'A is not well determined because the ENDOR transitions in the bc and c'a'planes lie within the 13-15-MHz envelope of distant proton resonances. The values of YAintand ?Aminwere obtained by assuming no significant anisotropy occurs in the bc plane. The g tensor also is lacking for accuracy because it is difficult to resolve one crystallographic site from the other in the ESR spectra taken off axis. The value of gisoagrees well with the value of 2.0033 reported by Madden and Fessenden12for the -CH20Rradical in an aqueous solution of methyl a-D-glucopyranoside at 27 "C. Also in good agreement are their reported isotropic values of Ya = 4.00 MHz and two equivalent values of = 5 1.24 MHz. The close similarity in these values and those reported here indicate that the .CH20R geometry in the aqueous phase is quite close to the geometry in the crystalline phase. ~
~
~
~~~~
~~~
(12) K. P. Madden and R. W. Fessenden, J . Am. Chem. SOC.,104, 2578 (1 982).
......
l l
/ /
' 0 .& .ee
\k
q q
......
n n
k
,
c 7... .. H y H y
......
BOND LENGTHS CI-HI .1.003,02I
c?-nr'.o.9m.m C7-01 -1,4358 1 1
1.400a
C7-01 Cl-01 -1,4358 Cl-01
l.400a
Figure 3. A diagram of the C H 2 0 Rradical geometry. The vector P is defined as making equal angles with the 01-C7, C7-H7, and C7-H7' bonds and is assumed to be parallel with the lone electron orbital axis.
Discussion Degree of Bending. The angle 8, used as a measure of nonplanarity, is defined in Figure 3 along with the other geometrical parameters discussed in this section. In order to calculate 0, two assumptions are necessary. One is that the principal axes a'A,ln and a2Amin are parallel to the respective C-H bonds, as is the case in planar 2pn radi~a1s.I~The other is that the 01-C7 bond does not deviate significantly from the orientation determined by the X-ray diffraction study. Under these assumptions the unit vector P, shown in Figure 3 as making equal angles with the three u-bonds, can be calculated as the vector making equal angles with lYl A,,,, 02Amln,and the 01-C7 bond. The resulting vector P (0.8714, -0.4897, -0.0281) makes an angle of 92.3 f 1.0" with the a-bonds, giving 8 = 2.3 f 1.Oo. In Figure 3 we have assumed that P coincides with the symmetry axis of the lone electron orbital (LEO). The angle 0 can be used to calculate the hybridization ratio A. If we use the formula14 A2 = c,2/c,z = 0.5 csc2 ( 8 ) - 1.5 with 0 = 2.3' the value of A2 is 309. For comparison, a tetrahedral configuration yields values of 0 = 19.47' and A2 = 3. The .CH20R radical is very nearly a pure 2pa radical. Atomic reorientation in forming the .CH20R radical from the parent molecule is demonstrably small. The angles given in Table (13) H. M. McConnell and J. Strathdee, Mol. Phys., 2, 129 (1959). (14) M . C. R. Symons and P. W. Atkins, "The Structure of Inorganic Radicals: An Application of Electron Spin Resonance to the Study of Molecular Structure", Elsevier, Amsterdam, 1967, p 257.
Bernhard et al.
1320 The Journal of Physical Chemistry, Vole88, No. 7, 1984 TABLE 11: Angles between Principal Axes of the CH,OR Radical and between Various Molecular Reference Vectors
Vla
v2a
v 1 . w angle,b deg
c747 C7-H7’
125.4 * 0.7 9.2 * 1.4 126.1 * 0.6 117.2 k 0.7 116.9 * 0.7 17.7 f 0.7 35.2 f 0.7
C7-H7”
33.0 ?r 0.7
‘VfAht cu2Aint gmin gmax
C7-01
15.2 f 1.4 14.0 * 1.4 27 f 27 82 f 20 89 k 6
C7---H1 01--R1
58 f 5 78 -c 4
“Amin “Aint “Amax
C7-01
C7-0 1
P P P
P gmax ‘Amax ‘Amax
a Molecular reference vectors are defined in Figure 3 method of error analysis see ref 11.
Baiso= p o ( ~ o+ B~ cosz e) has been evaluated for alkoxy radicals, yielding Bo = 14 MHz and B2 = 263 MHz.I7*’*The torsion angle 8 can be approximated by H1-C1-01-P by assuming the the unpaired electron centered on 01 is in a 2p orbital parallel to P. This gives 8 = 5 1 ’ and from ?Aiso= 6.1 MHz one obtains p o 0.05. The small unpaired electron population on H1 is most likely positive since ?Amaxis greater YAintz ?Amin. But for reasons that are not apparent to us the ?Amaxdirection does not correspond well with either the C7-H1 or 01-H1 directions (see Table 11). Bond Lengths. Gordy has derived a set of equations for calculating the three dipolar elements (By,B,, B,) of the “H hfc.I6 Using a two-center dipole approximation, B,, By, and B, are expressed in terms of p, R,, and R,. RH is the C-H bond length and R , is the distance from the LEO node to a point that serves as an effective center of each lobe of the 2p LEO. R, extends equal distances above and below the LEO nodal plane. Using two of these equations, here we use By and B,, one can solve for RH and R,. The resulting equations, giving RHand R, in A, are
-
For
I1 show the relative orientation of ESR-ENDOR measured directions and various molecular reference vectors. In order for the C7-H7 and C7-H7’ bonds to move into coincidence with “]Amin and “2Amin they need only reorient by 17.7’ and 35.2’, respectively. The H7” atom, assumed to be the one abstracted, forms a bond that would lie 33.0’ from P. More important one can use the sa2Alntangle and the “‘Aminsa2Amin angle to calculate the 01-C7-H7 and 01-C7-H7’ angles by assuming the later two angles are equal. This calculation, which is independent of crystallographic atomic coordinates, yields an angle of 116.9’ for both 01-C7-H7 and 01-C7-H,’. Comparison with the analogous angles *‘Am,,.01-C7 (1 17.2’) and “2A,i,.01-C7 (1 16.9’) indicates that the 0 1 x 7 bond does not shift significantly upon radical formation. It is notable that the H7-C7-H7’ angle (125.4’) is significantly larger than the other two inter u-bond angles. The sum of the three angles is 359.2’ which is less than 360.0’, as expected for a slightly bent structure. The dipolar components of an “H hfc tensor may be used to measure pc, the unpaired electron population of the LEO at the radical center.I5 This may be a more dependable means of determining the unpaired electron population than use of the isotropic component since the spin density in orbitals other than the adjacent LEO are heavily unweighted as contributors to the dipolar coupling due to the R-3 dependence on the distance R separating the nucleus and the unpaired electron. For “‘A and a2A the dipolar components (By,B,, B,) of the C H 2 0 R radical are (-34.2, 1.5, 32.8 MHz) and (-35.3, 3.8, 31.5 MHz), respectively. Gordy has surveyed a series of alkyl radicals and estimates that B, = 38.7 MHz for pc = 1.0.l6 Using this value and averaging the two values of B, obtained from the C H 2 0 R radical, one obtains pc = (32.8 31.5)/2(38.7) = 0.83. To estimate the unpaired electron population on 0 1 we assume that 01 is the only other major site of spin density. This gives po = 1 - pc = 0.17. One can try to estimate the value of po from ?Ah, treating the y hfc as though the isotropic part is transmitted via hyperconjugation between the C1-H1 bond and the unpaired electron on 01. The equation
+
(15) F.Q.Ngo, E. E. Budzinski, and H. C. Box, J . Chern. Phys., 60,3373 (1974). (16) W. Gordy, “Theory and Applications of Electron Spin Resonance”, Wiley, New York, 1980, 207-15.
- 7 8 . 6 8 ~ ’ I 3 2By R p = ( 7 )
+ B,
]I2
(F)
Substituting p = 0.83 and the minimum and maximum dipolar elements from *lA yields RH and R, corresponding to the C7-H7 bond. Likewise, the dipolar elements from “2A yields RH and R, corresponding to the C7-H7’ bond. For “‘A the results are RH = 1.00 f 0.02 A, R = 0.73 f 0.1 A and for “2A the results are R , = 0.98 f 0.02 R, = 0.75 f 0.01 A. We have calculated the values of RHand Rp,using the above equations, for over 50 different cy hfc tensors that have been obtained via ENDOR measurements. The results, which will appear elsewhere, are surprisingly consistent considering that a point dipole approximation is being employed over distances of only 1 8. The alkyl “H-C bonds contract by 0.05-0.10 8, upon free radical formation and the -CH20Rradical gives *H-C bond lengths typical for the alkyl radical group.
A,
Conclusions The .CH,OR radical in PMeGal is slightly bent with B = 2.3 1.0’, H-C-N’ = 125.4 f 0.7’, 0-C-H’ = 116.9 k 0.8’, and 0-C-H = 117.2 f 0.8’. The C-H bonds have contracted, perhaps as much as 0.1 A. The unpaired electron densities are pc 2 0.83 and po 5 0 . 1 7 . Acknowledgment. The authors thank Mr. Robert Spalletta for designing the computer interface and writing the software that greatly facilitated these experiments. This work was performed under the support of NSF Grant No. PCM77-16830 and partially under Contract No. DE-AC02-76EV03490 with The U S . Department of Energy at the University of Rochester Department of Radiation Biology and Biophysics and has been assigned Report NO. UR-3490-2291. Registry No. PMeGal, 1824-94-8; -CH,OR (R = @Gal),88766-65-8. (17) W. A. Bernhard, D. M. Close, M. J. Hiittermann, and H. Zehner, J . Chem. Phys., 67, 1211 (1977). (18) K. Toriyama and M. Iwasaki, J . Am. SOC.,101, 2516 (1979).
