Oxygen-17 Nuclear Quadrupole Double Resonance. 6. Effects of Hydrogen Bonding' Leslie G. Butler,2aC. P. Cheng,2band Theodore L. Brown* School of Chemical Sciences and Materials Research Laboratory, University of Iilinois, Urbana, Iiiinois 6180 1 (Received: May 18, 1981)
The 170nuclear quadrupole resonance (NQR) spectra in natural abundance of organic compounds having interand intramolecular hydrogen bonds in the solid state have been measured at 77 K via a field-cycling double-resonance technique. The compounds studied are 2-, 3-, and 4-hydroxybenzaldehyde;2-nitrobenzoic acid; 2-hydroxybenzoic acid; chloranilic acid; 1,4- and 1,8-dihydroxyanthraquinone;2-, 3-, and 4-nitrophenol; 2chlorophenol; catechol; 1-hydroxy-9-fluorenone;4(1H)-pyridinone;and benzoin. For comparison of the effect of hydrogen bonding upon the l'O NQR data, the following compounds were also studied: 9-fluorenone, benzyl benzoate, dibenzyl ether, and diphenyl ether. The effect of hydrogen bonding is evaluated by comparison of two structurally similar compounds, one of which exhibits hydrogen bonding, while the other does not. Hydrogen bonding produces a large effect upon the 1 7 0 electric field gradient at both hydrogen bond donor and acceptor sites. For short hydrogen bonds, a reduction in the I7O quadrupole coupling constant of 20% has been observed.
Introduction In the preceding papers in this series, we have reported 170(S = 5/2) nuclear quadrupole double-resonance spectra of a number of organic carbonyl compounds and substituted nitrobenzene^.^^ The results of this work suggest that the NQR data are quite sensitive to the effects of hydrogen bonding. To gain a greater understanding of the influence of hydrogen bonding upon the electric field gradient at both 1 7 0 and 2Hsites in an O-H-.O hydrogen bond, we have recently reported a theoretical study of the model hydrogen-bonding dimer, formaldehyde-watera7 In this paper, we report experimental NQR data on hydrogen bond systems used for comparison with theoretical calculations, together with data for several other systems. Experimental Section The techniques employed are essentially those described in the previous paper^.^" 4(1H)-Pyridinone was obtained from Professor Peter Beak. All other samples, examined as powdered solids or frozen liquids at 77 K, were obtained from commercial sources as materials of high purity and were used without further purification. It has been reported that 1,4-dihydroxyanthraquinoneexhibits two crystallographic modifications.8 Samples recrystallized from benzene, acetic acid, and ether revealed no substantial difference as determined by X-ray powder diffraction. The sample used for the NQR experiment was recrystallized from benzenesg Results The observed 170NQR frequencies in several oxygencontaining organic compounds are listed in Table I. As(1) This research was supported by the National Science Foundation through Research Grant DMR-77-23999 and by the National Institutes of Health, Institute of General Medicine, through Research Grant GM23395. (2) (a) Department of Chemistry, California Institute of Technology, Pasadena, CA 91125. (b) National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China. (3) Cheng, C. P.; Brown, T. L. J. Am. Chem. SOC.1979, 101, 2327. (4) Cheng, C. P.; Brown, T. L. Symp. Faraday SOC.1979, No. 13,75. (5) Cheng, C. P.; Brown, T. L. J. Am. Chem. SOC.1980, 102, 6418. (6) Hiyama, Y.; Brown, T. L. J. Phys. Chem. In press. (7) Butler, L. G.; Brown, T. L. J . Am. Chem. SOC.In press (paper 5 in this series). (8)Borgen, 0.Acta Chem. Scand. 1966, 20,2885. (9) Nigam, G. D.; Deppisch, B. Z . Kristallogr 1980,151, 185. 0022-3654/81/2085-2738$01.25/0
signment of the 170spectrum is generally straightforward; the techniques used for analysis of the NQR spectra have been p r e ~ e n t e d .Thus, ~ ~ only a few of the entries of Table I require comment. The 170NQR spectrum of 9fluorenone indicates the presence of two crystallographically nonequivalent oxygen sites, in agreement with the reported solid-state structure.1° As the four observed transitions could not be uniquely paired with the two sites, the values of the quadrupole coupling constant and asymmetry parameter reported in Table I are an average of the possible values. In cases where hydrogen bonding causes two otherwise identical oxygen sites to have different values for the quadrupole coupling constant and asymmetry parameter, the site involved in hydrogen bonding is assigned on the basis of the characteristic reduction in the value of the quadrupole coupling ~ o n s t a n t . ~ -For ~ example, in 1hydroxy-9-fluorenone, I, it is found, by comparison with H.
1
9-fluorenone, that the value of the quadrupole coupling constant of the carbonyl oxygen has decreased by 12% upon formation of a hydrogen bond (O.-O distance of 3.036 A),11 As found in the theoretical analysis of the formaldehyde-water interaction, this reduction is to be expected. As another example, in 4-nitrophenol, which has an intermolecular hydrogen bond,12 one can assign the transitions corresponding to the highest value of the quadrupole coupling constant to the non-hydrogen-bonded N-0 site. The more complicated 170NQR spectra of the compounds listed in Table I were assigned by means of similar considerations. In previous papers, we have applied the Townes-Dailey model13to the analysis of 170NQR data for organic carbonyl groups3 and singly connected oxygen bound to ni(10) Luss, H. R.; Smith, D. L. Acta Crystallogr., Sect. B 1972,28,8M. (11) Wilson, R.; Paul, I. C.; Curtin, D. Y., private communication. (12) Coppens, P.; Schmidt, G. M. J. Acta Crystallogr. 1965, 18, 62. (13) Townes, C. H.; Dailey, B. P. J. Chem. Phys. 1949, 17, 782.
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 19, 7981 2739
H-Bonding Effect on "0 NQR Spectra
TABLE I: Oxygen-17 NQR Data for both Hydrogen-Bonded and Non-Hydrogen-Bonded Compounds at 77 K comPd 2-hydroxy benzaldehyde
site
0-H
c=0 0-H c-0 0-H c=0
3-hydroxy benzaldehyde 4-hydroxy benzaldehyde
2-nitrobenzoic acid
C(0)OH C( 0)OH NO2 NO2 0-H C(O)OHC C(0)OH
2-hydroxybenzoic acid 2-nitrophenol 3-nitrophenol 4-nitrophenol
1,4-dihydroxyanthraquinone
1,8-dihydroxyanthraquinone
c=oc c=0 0-H c=0
chloranilic acid
0-H c= 0 c=0
1-hydroxy-9-fluorenone 9-fluorenone
0-H c=0 c=0
benzoin 4( 1H)-pyridinone benzyl benzoate
-0-
c=0 0-H
2-chlorophenol catechol
0-H 0-H -0-
dibenzyl ether diphenyl ether a
kilohertz.
megahertz.
0-H NO NO...H 0-H NO NO...H 0-H NO N0.a.H 0-H c=0 0-H 0-H
-0-
v[5/2 - 3/21a 2375 2922 2443 2920 2432 2880 2240 2502 3742 3814 2236 207 6 2390 2435 3764 3277 2471 3763 3409 2425 3687 3302 237 2 2888 2343 2376 2595 3122 2430 3012 2450 2920 1833 1770 2480 3035 2410 1639 2650 2382 2395 2337 2908 2863
v[3/2 - 1/2Ia 1575 (20) 1607 ( 3 ) 1830 (5) 1644 ( 3 ) 1576 (2) 1518 (1) 1251 ( 2 ) 2305 (1) 2560 (1) 2560 (1) 1670 (5) 1159 ( 2 ) 1303 ( 2 ) 1615 (1) 2553 (1) 2853 (1) 1891 (2) 2538 (1) 2669 ( 2 ) 1746 (2) 2603 (1) 2779 ( 2 ) 1510 (1) 1510 (1) 1480 (1) 1480 (1) 1300 (1) 1863 (1) 1470 (5) 1685 (1) 1720 (10) 1610 (10) 3018 (3) 3053 ( 3 ) 2480 (40) 1760 ( 3 ) 1500 (100) 1563 (5) 1335 ( 5 ) 1967 (1) 2112 ( 2 ) 2112 ( 2 ) 2725 (2) 2395 ( 2 )
e2Qq,,lhb 8.304 (22) 9.891 (14) 8.736 (7) 9.923 ( 8 ) 8.468 ( 3 ) 9.681 ( 2 ) 7.602 ( 5 ) 9.308 ( 2 ) 13.158 ( 2 ) 13.364 ( 2 ) 7.991 ( 8 ) 7.045 ( 3 ) 8.078 ( 5 ) 8.514 ( 2 ) 13.214 ( 2 ) 12.059 (1) 8.871 ( 3 ) 13.197 ( 2 ) 12.292 ( 2 ) 8.607 ( 2 ) 13.039 ( 2 ) 12.072 ( 3 ) 8.233 (2) 9.695 ( 2 ) 8.121 ( 2 ) 8.215 ( 2 ) 8.653 ( 2 ) 10.714 ( 2 ) 8.359 ( 8 ) 10.224 ( 2 ) 8.655 (15) 9.888 (16) 10.41 (5)
0.526 (17) 0.282 (4) 0.665 (4) 0.318 (5) 0.500 ( 3 ) 0.206 (2) 0.306 ( 6 ) 0.901 (1) 0.562 ( 8 ) 0.540 (1) 0.661 ( 6 ) 0.305 (4) 0.268 (6) 0.526 (1) 0.552 ( 2 ) 0.835 (2) 0.689 ( 2 ) 0.546 (1) 0.714 (1) 0.620 ( 2 ) 0.598 (2) 0.796 (2) 0.471 (1) 0.189 ( 3 ) 0.469 (1) 0.451 (1) 0.038 (1) 0.398 (1) 0.415 ( 7 ) 0.308 (2) 0.591 (11) 0.286 (15) 0.39 ( 3 )
9.37 ( 6 ) 10.365 ( 5 ) 8.33 (10) 6.139 ( 8 ) 8.844 ( 8 ) 8.677 ( 2 ) 8.835 ( 3 ) 8.662 ( 3 ) 10.855 ( 3 ) 10.455 ( 3 )
0.98 ( 2 ) 0.360 ( 6 ) 0.45 ( 9 ) 0.942 ( 8 ) 0.08 ( 4 ) 0.775 (1) 0.850 ( 2 ) 0.878 (2) 0.921 (2) 0.739 (2)
11
This oxygen site is involved in two hydrogen bonds.
TABLE 11: Townes-Dailey Analysis of I7O NQR Results for Singly Connected Oxygen compd 2-nitrobenzoic acid (N-0) 2-nitrophenol (N-0) 3-nitrophenol (N-0) 4-nitrophenol (N-0) 1,8-dihydroxyanthraquinone(C= 0) 9-fluorenone (C=O) benzyl benzoate (C=O) a
a' = 0.25 for C = 0 , 3 a z = 0.20 for
re1 efg components : z z ] z [xx [YY :zz1 z [ x x [YY :zz1 2 [ x x [YY :zz1 z [ x x [YY :zz1 2 [ x x [YY : x x ] z [zz [YY : x x ] 2 [zz [YY :zz1 > [ x x [YY : x x ] 2 [zz [YY
Pn 1.488 1.476 1.484 1.483 1.500 1.419 1.437 1.566 1.588
POa
1.066 1.057 1.064 1.067 1.065 1.407 1.422 1.451 1.451
N-0 bonds.5
trogen, sulfur, or p h o s p h ~ r u s .The ~ data in Table I that pertain to singly connected oxygen sites which are not perturbed by hydrogen bonding are suitable for a similar analysis. The description of the oxygen valence orbitals used for the Townes-Dailey analysis and the molecular axis system (shown in Figure 1) are the same as used earlier.3*s The results are presented in Table 11. For all of the compounds listed in Table 11, the analysis suggests that the sign of the quadrupole coupling constant is positive and the principal axis of the electric field gradient is aligned along the y axis of the molecular axis system. Thus, these compounds satisfy the requirements for comparison with the results of the theoretical calcu-
lation of the formaldehyde-water dimer. The results of alternative assumptions regarding the orientation of the electric field gradient tensor axes in benzyl benzoate by the Townes-Dailey analysis do not lead to a unique assignment. This is not unexpected as Cheng and Brown have predicted that a change in the axis orientation should occur at p r = 1.55.3
Discussion Theoretical analysis of a model hydrogen-bonding system leads to the prediction that formation of an 0-H-0 hydrogen bond results in decreases in the quadrupole coupling constants of both the donor and acceptor oxygen
2740
J. Phys. Chem. 1981, 85,2740-2746
(Y)
Y
(Z) Flgure 1. Axis system in C=O and N-0 bonds: X = C, N.
atoms, as compared with the values for otherwise similar but non-hydrogen-bonded atoms.7 The experimental data presented here corroborate the theoretical results (see also Figure 2 of ref 7 ) . In addition to the direct study of hydrogen bonding in the form of geometrical interpretation of the I7O NQR data, the results presented here and in the previous paper facilitate the analysis of the electronic effects that are reflected in the I7O NQR data. For example, the carbonyl group in 4(1H)-pyridinone is structurally similar to that in p-benzoquinone, but has a quite different quadrupole
coupling constant, 8.33 vs. 11.51 MHz.14 A large portion of the difference in the values of the quadrupole coupling constant is probably due to the effect of hydrogen bonding in 4(1H)-pyridinone, although the solid-state structure of this compound is not known. If we assume that the length of the hydrogen bond is the same as is found in 2(1H)pyridinone (2.77 A),'5 then we can estimate the effect of hydrogen bonding on the 170quadrupole coupling constant to be about 1.0 MHz. Because this still leaves a large difference of 2.2 MHz between the two oxygen sites, one can infer that there exists a significant difference in the electronic environment of the two carbonyl groups. The lower quadrupole coupling constant in 4(1H)-pyridinone suggests a much larger oxygen 2p, population. In summary, we have presented 1 7 0 NQR data for several hydrogen-bonded systems. The experimental data support the results of a previous theoretical study. The results demonstrate that hydrogen bonding in the solid state is amenable to study by 170NQR techniques. Secondly, the effects of hydrogen bonding upon the electric field gradient can be estimated, thus aiding in the further analysis of intramolecular electronic effects on the 1 7 0 NQR data. ~
~~~~~
(14)Hsieh,Y.;Koo, J. C.; Hahn, E. L. Chem. Phys. Lett. 1972,13,663. (15)Penfold, B.R.Acta Crystallogr. 1953,6, 591.
Gas-Phase Hydroxyl Radical Reactions. Products and Pathways for the Reaction of OH with Aromatic Hydrocarbons' Richard A. Kenley," John E. Davenport, and Dale 0. Hendry' Physical Organic Chemistry DepaHment, Chemlstry Laboratory, SRI International, Menlo Park, California 94025 (Received: August 5, 1980; In Final Form: May 26, 1981)
The reactions of OH with benzene, toluene, 1,4-dimethylbenzene,and 1,3,5-trimethylbenzenehave been investigated in a discharge flow system at total pressures of 6-12 torr. In the presence of NO, NOz, and O2the intermediate radicals are converted quantitatively to stable products from which two major reaction pathways are identified. Abstraction of a hydrogen atom from the methyl group by OH leads primarily to the corresponding aldehyde, and addition to the aromatic ring by OH gives primarily the corresponding phenols. Nitro substitution of the aromatics via the OH adduct competes with phenol formation; however, the amount depends on the ratio of NOz/Oz. In the case for toluene, the ratio of rate constants, kNOf/kol, for the two processes is (4f 2) X lo3. The isomer distributions for OH addition to toluene were determined to be 0.806 f 0.022 ortho, 0.051 f 0,009 meta, 0.143 & 0.019 para, and
