J. Phys. Chem. 1987, 91, 6592-6595
6592
ARTICLES Bendtng in the *CH,OR Radical Trapped in Crystals of Methyl a-D-Mannopyranoside As Measured by ENDOR William A. Bernhard* and Kermit R. Mercer Department of Biophysics, The University of Rochester, Rochester, New York 14642 (Received: March 16, 1987; In Final Form: June 10, 1987)
A -CH20Rradical trapped in single crystals of methyl a-Pmannopyranoside has been characterized by ENDOR measurements at 77 K. The highly accurate a-hydrogen tensors are used to calculate the geometry about the free-radical center. The .CH20R radical is bent with 8 = 5.8 f 0.4O, where 0 is the angle between the nodal plane of the lone electron orbital and the three (r bonds (0-C, H-C, H'-C). The three angles H-C-H', 0-C-H, and 0-C-H' are 127.0 f 0.4', 108.0 k 0.2', and 122.1 f 0.2", respectively. The unpaired electron densities are pc N 0.83 and po N 0.17.
fO.1' with the precession camera. The accuracy was verified by the ENDOR data. The ENDOR data were collected with a home-built cryotip cavity, described previously,6 set into a modified IBM-Bruker 2OOD-SRC ESR spectrometer operating at X-band microwave frequencies. The ENDOR rf source (an Ailtech 380, Eaton Co.) was modulated by an EG&G PAR lock-in (Model 5207) a t 10 kHz with deviations between 50 and 150 kHz. Power amplification of the rf was achieved with a 50-W EN1 3502 amplifier. Data collection, storage, and plotting were controlled by an IBM PC/XT augmented with a Data Translation A/D board OH OH \ \\ (DT2801), a Capital Equipment Corp. GPIB board (PC488), and use of software written in our laboratory. Ho\ H6 A H 6 Analysis of the ENDOR data utilized a nonlinear least-squares method described previ~usly.~As in the study of PMeGal, the tensors and variance-covariance elements were determined from HO HO H5 H5 O1?H73 H71 ten data sets, two sets from the two magnetically distinct sites \ OH Hi in each of the three planes bc, ca, and ab and four sets from the H3 skew plane ba' The two a-hydrogen hfc tensors were correlated \ 37H' to the same crystallographic site by observing which ENDOR .CH,OR in aMEMAN H72 'CH20R in #MEGAL transitions were associated with a particular portion of the ESR spectrum when Ho was between axes. We were unable to site Other reports on the degree of bending in oxyalkyl radicals are briefly reviewed in our previous paper.' The earliest of t h e ~ e , ~ . ~ correlate the y and c hfc tensors because the site splitting was too small. using 13Chyperfine splitting, has provided an estimate for bending The accuracy of the tensors determined in aMeMan exceeds in - C H 2 0 Hof -4O, in accord with the ENDOR measurements that of the already very accurate tensors determined in PMeGal. presented herein. This is the most precise data set collected in our laboratory. There are two reasons for the improvement. One is that the crystals Materials and Methods are now translated onto the ESR pedestals with a more precise Polycrystalline methyl a-D-mannopyranoside was purchased translator. The other is that the ENDOR line widths in aMeMan from Sigma Chemical Co. and Aldrich Chemical Co. Crystals are narrower than usual, measuring between 60 and 80 kHz. The were grown from H 2 0and D 2 0 by either evaporation or cooling reduced line width may be inherent in the sample type, but we between 50 and 5 OC. By use of a Buerger precession camera, suspect that with our new ENDOR configuration there is less of the crystals were verified to have the reported space group of a tendency to modulation broaden the ENDOR transitions. The P21212 and unit cell dimensions of a = 9.43, b = 9.31, and c = mounting accuracy is of particular importance because it reduces 10.05 the chances of systematic errors that do not show up in the analysis Alignment of the crystals for ENDOR data collection was done of statistical error. by using a precession camera. Data were collected in four planes: ab, bc, ca, and a skew plane ba'where a'is 42.0' from a in the Results and Analysis ca plane. The orientation of each axis was determined to within Single crystals of aMeMan were X-irradiated at 77 K to a dose of 7 Mrad, and ENDOR measurements were made at 77 K. The (1) Bernhard, W. A.; Homing, T. L.; Mercer, K. R. J. Phys. Chem. 1984, ESR spectrum of the .CHIOR radical in aMeMan is the same 88, 1317. (2) Fessenden, R. W.; Schuler, R. H. J. Chem. Phys. 1965, 43, 2704. as in BMeGal, but in aMeMan there is considerable ESR intensity
Introduction In an earlier report ENDOR was used to measure the bending of the C H 2 0 R radical in methyl 8-D-galactopyranoside (PMeGal).' The bending angle 8, defined as the angle between each of the three a-bonds and the plane normal to the lone electron orbital (LEO), was determined to be 2.3 f 1.0'. Trapping of the .CH20R radical in methyl a-D-mannopyranoside (aMeMan) has afforded the opportunity to examine the degree of bending in a molecule that varies slightly in structure and chemical environment. The two radical structures are as follows:
k4,5
(3) Fessenden, R. W. J. Phys. Chem. 1967, 71, 74. (4) Jeffrey, G. A.; McMullan, R. K.; Takagi, S. Acta Crystallogr., Sect. B 1977, B33, 728. (5) Gatehouse, B. M.; Poppleton, B. J. Acta Crystallogr., Sect. B 1970, B26, 1761.
0022-3654/87/2091-6592$01 SO10
(6) Close, D. M.; Fouse, G. W.; Bernhard, W. A. J. Chem. Phys. 1977, 66, 1534. (7) Fouse, G. W.; Bernhard, W. A. J. Magn. Reson. 1978, 32, 191.
0 1987 American Chemical Society
.CHIOR Radical Trapped in aMeMan Crystals
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6593
TABLE I: Hyperline Coupling Tensors for the .CH,OR Radical
principal
variance-covariance matrixCx io5
principal axisb
label
value," MHz
I
m
n
alAma, dlAint alAmin a'Aim
91.28 (5) 53.95 (7) 23.52 (5) 56.3 (1)
-0.6257 -0.7775 0.0630
0.0062 -0.0858 -0.9963
02
A,,, a2Aint u2Amin a2Aiso
85.98 (7) 47.52 (6) 19.26 (8) 50.9 (1)
0.4032 -0.8053 -0.4347
YA,, 'Aint 'Amin 'Ais0
14.90 (3) 3.92 (3) 3.29 (4) 7.37 (7)
