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Nitrogen-14 nuclear quadrupole resonance spectra of substituted nitrobenzenes
JOURNAL
OF MAGNETIC
RESONANCE
28,
391-395 (1977)
Nitrogen- 14 Nuclear Quadrupole ResonanceSpectra of Substituted Nitrobenzenes C.P. CHENGANDTHEODORE L. BROWN* School
of Chemical
Sciences
and Materials Research Laboratory, Urbana, Illinois 61801
University
of Illinois,
Received April 26, 1977 The 14N nuclear quadrupole resonance spectra have been measured for nitrobenzene and nine substituted nitrobenzenes. The experiments were carried out at 77 K, using the double-resonance, level-crossing technique. The 14N signals for the -NH, groups of mand p-nitroaniline were also observed. The quadrupole coupling constants for the nitro group nitrogens vary from 1493 kHz for p-nitroaniline (II = 0.292) to 1232 kHz for mdinitrobenzene (r] = 0.360). The dependence of e2Qq/h upon the meta or the para substituent suggests that the substituents affect mainly the covalency in the nitrogencarbon o bond. A linear relationship is observed between e*Qq/h and the Hammett substituent constants. INTRODUCTION
Although the nitro group is an important functional group in organic chemistry only one study of the i4N NQR spectra of the nitro group in organic nitro compounds has been reported (I). The lack of such data is the result of a low field gradient at nitrogen, with correspondingly low quadrupole transition frequencies. Low-frequency pure NQR transitions in solid samples are not readily seen with conventional pulse or cw techniques. We report here the 14N NQR spectra of several substituted nitrobenzenes, obtained using the double-resonance, level-crossing technique (2-5). This method is sensitive, and especially useful for observing very low frequency transitions. EXPERIMENTAL
The compounds examined were all Eastman Kodak materials, used without further purification. Our experimental arrangement has been described elsewhere (4, 5). All spectra were obtained at 77 K. RESULTS AND DISCUSSION
The 14N NQR spectra for nitrobenzene and nine substituted nitrobenzenes are listed in Table 1. In addition, the i4N transitions for the -NH, groups of m- and p-nitroaniline are also listed. The values of v, and V- for nitrobenzene are the same as those reported by Subbarao et al. (I). In all of the compounds studied, only one crystallographically distinguishable 14N resonance set is seen These results are in agreement with the known crystal structures for nitrobenzene (6), m-chloronitrobenzene (7), p-aminonitrobenzene (8), mbromonitrobenzene (9), m-dinitrobenzene (IO), and p-dinitrobenzene (II). It is * To whom correspondence should be addressed. Copyright 0 1977 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
391 ISSN 0022-2364
392
CHENG AND BROWN TABLE 14N
NQR
DATA
FOR NITROBENZENES
Compound p-Nitroaniline (-“‘NH*) m-Nitroaniline (J4NHz) Nitrobenzene p-Iodonitrobenzene m-Nitrobenzaldehyde p-Chloronitrobenzene p-Nitrobenzaldehyde m-Chloronitrobenzene p-Dinitrobenzene m-Dinitrobenzene
1
v+
V-
1229 2943 1226 3216 1213 1146 1114 1125 1070 1066 1036 1035
1011 2434 954 2590 925 898 872 857 856 840 833 813
AT 77
Ka
VII
e=Qslh
rl
1493 3585 1453 3871 1425 1363 1324 1321 1284 1271 1246 1232
0.292 0.284 0.374 0.323 0.404 0.364 0.366 0.406 0.333 0.356 0.326 0.360
218 509 272 626 288 248 242 268 214 226 203 222
u The transition frequencies v,, v-, and V, and quadrupole coupling constants e2Qq/h kilohertz. b Reference (16).
aL -0.66 -0.15 0 0.28 0.38 0.23 0.22 0.37 0.78 0.71 are given in
interesting that in certain of the nitrobenzene structures the plane of the nitro group is twisted with respect to the aromatic ring. For example, in the m-dinitrobenzene, the nitro groups are twisted about 13” from the ring plane. In nitrobenzene itself the O-N-O angle is 124O. The angle appears to be slightly larger than this in the compounds in which electron-withdrawing groups are also found on the ring. For example, in m-dinitrobenzene the angle is about 126”, and in p-dinitrobenzene, about 127’. On the other hand, in N,N-dimethyl-p-nitroaniline, the O-N-O angle is 12 1o (12). The only significant departure of the O-N-O angle from about 124O is reported for m-chloronitrobenzene (7). However, this does not appear to have been a very accurate structure determination. The 14N quadrupole coupling constants are in the range 1.2 to 1.5 MHz. They exhibit a substantial dependence on the meta or the para substituent of the phenyl ring. It is possible to understand this dependence in terms of a modified Townes-Dailey model (13) which describes the field gradient at 14N in terms of the populations of the valence 2p orbitals. The geometry at the nitro group in nitrobenzene is illustrated in Fig. 1. The C-N bond is taken to lie along the z axis. The axis system shown in Fig. 1 serves also to Y
0 2
\/
N \ \
FIG.
tensor.
0 X
1. Orientation of the local axis system used to define the major components of the field gradient
NQR
OF
NITROBENZENES
393
describe the orientation of the orbitals in the nitrite, (NO,-) ion. In the NO,- ion, the nitrogen orbital that projects along the z axis contains an unshared electron pair that generates a large field gradient at nitrogen (14). The orientation and general character of the field gradient tensor at nitrogen in the NO,- ion is thus not unlike that observed in pyridine, in which the field gradient tensor is also dominated by the unshared electron pair (5, 1.5). Application of the Townes-Dailey model to account for the electron distribution in NO,- is entirely analogous to the application to pyridine. The covalent bonding to the ring carbon atom in nitrobenzene results in a substantially decreased population of the nitrogen orbital directly along the z axis, as compared with NO,-. Thus the field gradient at 14N is much lower in the nitrobenzenes than in the nitrite ion. In addition, the population of the p, orbital on nitrogen could be affected as a result of a rc interaction between the nitro group and the phenyl ring 7c system. The components of the field gradient tensor at nitrogen in the axis system shown in Fig. 1 can be expressed in terms of the populations of the hybrid orbitals consistent with the observed bond angles and the planar trigonal environment of the nitrogen. For this purpose we assume an O-N-O angle of 124”, the observed angle in nitrobenzene itself. The major components of the field gradient tensor are given by the expressions in the equations qyy = q. (-0.35861-
0.64140 + rc),
q,, = q,, (-0.35862 + 0.8587~ - n/2), qz2= q. (0.71731-
111
0.21730 -n/2).
In these equations, I represents the population of the nitrogen orbital directed along the z axis (the lone-pair orbital in the NO,- ion or the C-N bond in nitrobenzenes); u represents the nitrogen atom populations in each of the hybrid orbitals which form the u bonds to oxygen; 7~represents the occupation of thep, orbital normal to the plane of the -NO, group. The symbol q. represents the field gradient generated by a single electron in a valence 2p orbital of nitrogen. The experimentally determined 14N quadrupole transition frequencies, v,, Y-, and v, are related to the diagonal components of the field gradient tensor by Eqs. [2]. The qxx, qru, and qzz are related to q,, qyy and q,, of Eqs. [ll simply by relabeling, to maintain thecondition Iq,,l > Iq,,l > Iq,l. q.d%=
CW3e2Qso> (v+ + v-1,
qyylqo =
(W3e2Qq0)(-2v+ + F>,
[21
qxx/q,, = GWe*Qq,,) (v, - 2~). In applying Eq. [I] to the nitrite ion or pyridine (IS) it may be assumed that 1 is 2. The observed quadrupole transitions may thus be employed to fix the two independent variables u and 7cafter assuming a suitable value for e*Qq,,/h. In the nitrobenzenes the number of unknowns exceeds the number of observables, since the occupancy of the nitrogen orbital directed toward carbon is also unknown. It seems reasonable to assume that I > o on the grounds that the effective electronegativity of the carbon atom of the phenyl ring should be less than that of the oxygen atoms in the nitro group. From their
394
CHENG AND BROWN
analysis of the NO,- data, Marino and Bray (14) conclude that z > u. On the other hand, Subbarao et al. (1) have assumed that in C-nitro compounds, 1 > u > 7~. Although it is not possible to tell with any reliability the relative values of 1, 7c,and 0, the observed effect of ring substituents on the field gradient parameters does help to limit the possibilities. It is evident from the data in Table 1 that electron-releasing substituents in the ring result in an increase in e*Qq/h, whereas electron-withdrawing substituents, such as Cl or NO,, result in a decrease in e2Qq/h relative to nitrobenzene itself. Meta or para ring substituents, especially those that are electron withdrawing,
FIG. 2. Variation in quadrupole coupling constant, @Qq/h, constant in meta- and para-substituted nitrobenzenes.
as a function of the Hammett substituent
should affect the field gradient at nitrogen mainly through their effect on 1. An electronreleasing substituent should increase the value for 1, whereas electron-withdrawing substituents should have the opposite effect. The effect of substituents on e2Qq/h is reproduced for the case in which 1 > 71> o. For example, for I = 1.20, 7c= 1.00 and IJ = 0.97, qxx/qo, q,,,,/q,,, and q,,/qo have values -0.0974, -0.0525, and 0.1500, respectively. This means that the major axis of the field gradient tensor is directed along the z axis of Fig. 1. Assuming e2QqJh = 9.0 MHz, these values correspond to e2Qq/h = 1.35 MHz, q = 0.30. Although these numbers fall in the range of the observed field gradient tensors for the nitrobenzenes, they are given merely for the sake of illustration. With this choice of relative values for q,,, q,,,,, and q,,, an increase in the value of 1 results in an increased value for e2Qq/h. It is consistent with this picture that e2Qq/h is substantially greater for CH,NO, than for C,H,NO, (1) whereas q is not much altered. However, our assignment of principal axes differs from that based on nuclear magnetic relaxation rate measurements (17). The effect of ring substituents on the field gradient parameters of the nitro group nitrogen can be shown by graphing e2Qq versus the Hammett u parameter for the meta or para substituent, as illustrated in Fig. 2. The data for a variety of substituents describe a reasonably good linear relationship. The most notable exception is the datum for p-nitroaniline. The crystal structure for this compound (8) shows extensive hydrogen
NQR
395
OF NITROBENZENES
bonding between the oxygen atoms of one molecule and the amino hydrogens of an adjacent molecule. The structure consists of linear arrays of hydrogen-bonded molecules. ACKNOWLEDGMENTS This
research
The Materials
was Research
supported
by the National
Laboratory,
University
Science
Foundation
of Illinois,
and Research
through
Grant
Grant
CHE
DMR-7601058
with
76-17570.
REFERENCES 1. S. N. SLJBBARAO, of nitro groups 2. R. 3. D. 4. Y. 5. Y.
E. T. N. N.
E. G. SAUER, AND P. J. BRAY, Phys. Lett. A 42,461 (1973). is in press: S. N. SUBBARAO AND P. J. BRAY, J. Chem. Phys.,
A more extensive in press.
SLUSHER AND E. L. HAHN, Phys. Rev. 166,332 (1968). EDMONDS, Pure Appl. Chem. 40,193 (1974). HSIEH, P. S. IRELAND, AND T. L. BROWN, J. h4ugn. Resonance 21,445 (1976). HSIEH, G. V. RUBENACKER, C. P. CHENG, AND T. L. BROWN, J. Amer. Chem.
(1977). 6. J. TROTTER, Acta Crystallogr. 12,884 7. E. M. GOPALAKRISHNA, Z. Kristallogr.
Sot.
study
99, 1384
(1959).
121,378
(1965).
Crystallogr. 14, 1009 (1961). 9. T. L. CHARLTON AND J. TROTTER,&& Crystallogr. 16,613 (1963). 10. J. TROTTER AND C. S. WILLISTON, Acta Crystallogr. 21,285 (1966). II. J.TROTTER, CQ~U~. J. Chem. 39,1638 (1961). 12. T. C. W. MAK AND J. TROTTER, Acta Crystallogr. l&68 (1965). 13. C. H. TOWNES AND B. P. DAILEY, J. Chem. Phys. 17,782 (1949); 20,35 (1952). 14. R. A. MARMO AND P. J. BRAY, J. Chem. Phys. 48,4833 (1968). 8. K. N. TRUEBLOOD,
15.
G. GOLDFISH,
AND J. DONAHUE,ACtU
16.
E. SCHEMPP AND P. J. BRAY, in “Physical Chemistry, An Advanced Vol. IV, Chap. 11, Academic Press, New York, 1970. (a) R. W. TAFT, JR., N. C. DENO, AND P. S. SKELL, Ann. Rev. Phys.
17.
JAFFE, Chem. Rev. 53,191 (1953). R. E. STARK, R. L. VOLD, AND R. R. VOLD,
Chem.
Phys.
20,337
(1977).
Treatise”
Chem.
(D.
Henderson,
9, 287 (1958);
Ed.),
(b) H. H.
OF MAGNETIC
RESONANCE
28,
391-395 (1977)
Nitrogen- 14 Nuclear Quadrupole ResonanceSpectra of Substituted Nitrobenzenes C.P. CHENGANDTHEODORE L. BROWN* School
of Chemical
Sciences
and Materials Research Laboratory, Urbana, Illinois 61801
University
of Illinois,
Received April 26, 1977 The 14N nuclear quadrupole resonance spectra have been measured for nitrobenzene and nine substituted nitrobenzenes. The experiments were carried out at 77 K, using the double-resonance, level-crossing technique. The 14N signals for the -NH, groups of mand p-nitroaniline were also observed. The quadrupole coupling constants for the nitro group nitrogens vary from 1493 kHz for p-nitroaniline (II = 0.292) to 1232 kHz for mdinitrobenzene (r] = 0.360). The dependence of e2Qq/h upon the meta or the para substituent suggests that the substituents affect mainly the covalency in the nitrogencarbon o bond. A linear relationship is observed between e*Qq/h and the Hammett substituent constants. INTRODUCTION
Although the nitro group is an important functional group in organic chemistry only one study of the i4N NQR spectra of the nitro group in organic nitro compounds has been reported (I). The lack of such data is the result of a low field gradient at nitrogen, with correspondingly low quadrupole transition frequencies. Low-frequency pure NQR transitions in solid samples are not readily seen with conventional pulse or cw techniques. We report here the 14N NQR spectra of several substituted nitrobenzenes, obtained using the double-resonance, level-crossing technique (2-5). This method is sensitive, and especially useful for observing very low frequency transitions. EXPERIMENTAL
The compounds examined were all Eastman Kodak materials, used without further purification. Our experimental arrangement has been described elsewhere (4, 5). All spectra were obtained at 77 K. RESULTS AND DISCUSSION
The 14N NQR spectra for nitrobenzene and nine substituted nitrobenzenes are listed in Table 1. In addition, the i4N transitions for the -NH, groups of m- and p-nitroaniline are also listed. The values of v, and V- for nitrobenzene are the same as those reported by Subbarao et al. (I). In all of the compounds studied, only one crystallographically distinguishable 14N resonance set is seen These results are in agreement with the known crystal structures for nitrobenzene (6), m-chloronitrobenzene (7), p-aminonitrobenzene (8), mbromonitrobenzene (9), m-dinitrobenzene (IO), and p-dinitrobenzene (II). It is * To whom correspondence should be addressed. Copyright 0 1977 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
391 ISSN 0022-2364
392
CHENG AND BROWN TABLE 14N
NQR
DATA
FOR NITROBENZENES
Compound p-Nitroaniline (-“‘NH*) m-Nitroaniline (J4NHz) Nitrobenzene p-Iodonitrobenzene m-Nitrobenzaldehyde p-Chloronitrobenzene p-Nitrobenzaldehyde m-Chloronitrobenzene p-Dinitrobenzene m-Dinitrobenzene
1
v+
V-
1229 2943 1226 3216 1213 1146 1114 1125 1070 1066 1036 1035
1011 2434 954 2590 925 898 872 857 856 840 833 813
AT 77
Ka
VII
e=Qslh
rl
1493 3585 1453 3871 1425 1363 1324 1321 1284 1271 1246 1232
0.292 0.284 0.374 0.323 0.404 0.364 0.366 0.406 0.333 0.356 0.326 0.360
218 509 272 626 288 248 242 268 214 226 203 222
u The transition frequencies v,, v-, and V, and quadrupole coupling constants e2Qq/h kilohertz. b Reference (16).
aL -0.66 -0.15 0 0.28 0.38 0.23 0.22 0.37 0.78 0.71 are given in
interesting that in certain of the nitrobenzene structures the plane of the nitro group is twisted with respect to the aromatic ring. For example, in the m-dinitrobenzene, the nitro groups are twisted about 13” from the ring plane. In nitrobenzene itself the O-N-O angle is 124O. The angle appears to be slightly larger than this in the compounds in which electron-withdrawing groups are also found on the ring. For example, in m-dinitrobenzene the angle is about 126”, and in p-dinitrobenzene, about 127’. On the other hand, in N,N-dimethyl-p-nitroaniline, the O-N-O angle is 12 1o (12). The only significant departure of the O-N-O angle from about 124O is reported for m-chloronitrobenzene (7). However, this does not appear to have been a very accurate structure determination. The 14N quadrupole coupling constants are in the range 1.2 to 1.5 MHz. They exhibit a substantial dependence on the meta or the para substituent of the phenyl ring. It is possible to understand this dependence in terms of a modified Townes-Dailey model (13) which describes the field gradient at 14N in terms of the populations of the valence 2p orbitals. The geometry at the nitro group in nitrobenzene is illustrated in Fig. 1. The C-N bond is taken to lie along the z axis. The axis system shown in Fig. 1 serves also to Y
0 2
\/
N \ \
FIG.
tensor.
0 X
1. Orientation of the local axis system used to define the major components of the field gradient
NQR
OF
NITROBENZENES
393
describe the orientation of the orbitals in the nitrite, (NO,-) ion. In the NO,- ion, the nitrogen orbital that projects along the z axis contains an unshared electron pair that generates a large field gradient at nitrogen (14). The orientation and general character of the field gradient tensor at nitrogen in the NO,- ion is thus not unlike that observed in pyridine, in which the field gradient tensor is also dominated by the unshared electron pair (5, 1.5). Application of the Townes-Dailey model to account for the electron distribution in NO,- is entirely analogous to the application to pyridine. The covalent bonding to the ring carbon atom in nitrobenzene results in a substantially decreased population of the nitrogen orbital directly along the z axis, as compared with NO,-. Thus the field gradient at 14N is much lower in the nitrobenzenes than in the nitrite ion. In addition, the population of the p, orbital on nitrogen could be affected as a result of a rc interaction between the nitro group and the phenyl ring 7c system. The components of the field gradient tensor at nitrogen in the axis system shown in Fig. 1 can be expressed in terms of the populations of the hybrid orbitals consistent with the observed bond angles and the planar trigonal environment of the nitrogen. For this purpose we assume an O-N-O angle of 124”, the observed angle in nitrobenzene itself. The major components of the field gradient tensor are given by the expressions in the equations qyy = q. (-0.35861-
0.64140 + rc),
q,, = q,, (-0.35862 + 0.8587~ - n/2), qz2= q. (0.71731-
111
0.21730 -n/2).
In these equations, I represents the population of the nitrogen orbital directed along the z axis (the lone-pair orbital in the NO,- ion or the C-N bond in nitrobenzenes); u represents the nitrogen atom populations in each of the hybrid orbitals which form the u bonds to oxygen; 7~represents the occupation of thep, orbital normal to the plane of the -NO, group. The symbol q. represents the field gradient generated by a single electron in a valence 2p orbital of nitrogen. The experimentally determined 14N quadrupole transition frequencies, v,, Y-, and v, are related to the diagonal components of the field gradient tensor by Eqs. [2]. The qxx, qru, and qzz are related to q,, qyy and q,, of Eqs. [ll simply by relabeling, to maintain thecondition Iq,,l > Iq,,l > Iq,l. q.d%=
CW3e2Qso> (v+ + v-1,
qyylqo =
(W3e2Qq0)(-2v+ + F>,
[21
qxx/q,, = GWe*Qq,,) (v, - 2~). In applying Eq. [I] to the nitrite ion or pyridine (IS) it may be assumed that 1 is 2. The observed quadrupole transitions may thus be employed to fix the two independent variables u and 7cafter assuming a suitable value for e*Qq,,/h. In the nitrobenzenes the number of unknowns exceeds the number of observables, since the occupancy of the nitrogen orbital directed toward carbon is also unknown. It seems reasonable to assume that I > o on the grounds that the effective electronegativity of the carbon atom of the phenyl ring should be less than that of the oxygen atoms in the nitro group. From their
394
CHENG AND BROWN
analysis of the NO,- data, Marino and Bray (14) conclude that z > u. On the other hand, Subbarao et al. (1) have assumed that in C-nitro compounds, 1 > u > 7~. Although it is not possible to tell with any reliability the relative values of 1, 7c,and 0, the observed effect of ring substituents on the field gradient parameters does help to limit the possibilities. It is evident from the data in Table 1 that electron-releasing substituents in the ring result in an increase in e*Qq/h, whereas electron-withdrawing substituents, such as Cl or NO,, result in a decrease in e2Qq/h relative to nitrobenzene itself. Meta or para ring substituents, especially those that are electron withdrawing,
FIG. 2. Variation in quadrupole coupling constant, @Qq/h, constant in meta- and para-substituted nitrobenzenes.
as a function of the Hammett substituent
should affect the field gradient at nitrogen mainly through their effect on 1. An electronreleasing substituent should increase the value for 1, whereas electron-withdrawing substituents should have the opposite effect. The effect of substituents on e2Qq/h is reproduced for the case in which 1 > 71> o. For example, for I = 1.20, 7c= 1.00 and IJ = 0.97, qxx/qo, q,,,,/q,,, and q,,/qo have values -0.0974, -0.0525, and 0.1500, respectively. This means that the major axis of the field gradient tensor is directed along the z axis of Fig. 1. Assuming e2QqJh = 9.0 MHz, these values correspond to e2Qq/h = 1.35 MHz, q = 0.30. Although these numbers fall in the range of the observed field gradient tensors for the nitrobenzenes, they are given merely for the sake of illustration. With this choice of relative values for q,,, q,,,,, and q,,, an increase in the value of 1 results in an increased value for e2Qq/h. It is consistent with this picture that e2Qq/h is substantially greater for CH,NO, than for C,H,NO, (1) whereas q is not much altered. However, our assignment of principal axes differs from that based on nuclear magnetic relaxation rate measurements (17). The effect of ring substituents on the field gradient parameters of the nitro group nitrogen can be shown by graphing e2Qq versus the Hammett u parameter for the meta or para substituent, as illustrated in Fig. 2. The data for a variety of substituents describe a reasonably good linear relationship. The most notable exception is the datum for p-nitroaniline. The crystal structure for this compound (8) shows extensive hydrogen
NQR
395
OF NITROBENZENES
bonding between the oxygen atoms of one molecule and the amino hydrogens of an adjacent molecule. The structure consists of linear arrays of hydrogen-bonded molecules. ACKNOWLEDGMENTS This
research
The Materials
was Research
supported
by the National
Laboratory,
University
Science
Foundation
of Illinois,
and Research
through
Grant
Grant
CHE
DMR-7601058
with
76-17570.
REFERENCES 1. S. N. SLJBBARAO, of nitro groups 2. R. 3. D. 4. Y. 5. Y.
E. T. N. N.
E. G. SAUER, AND P. J. BRAY, Phys. Lett. A 42,461 (1973). is in press: S. N. SUBBARAO AND P. J. BRAY, J. Chem. Phys.,
A more extensive in press.
SLUSHER AND E. L. HAHN, Phys. Rev. 166,332 (1968). EDMONDS, Pure Appl. Chem. 40,193 (1974). HSIEH, P. S. IRELAND, AND T. L. BROWN, J. h4ugn. Resonance 21,445 (1976). HSIEH, G. V. RUBENACKER, C. P. CHENG, AND T. L. BROWN, J. Amer. Chem.
(1977). 6. J. TROTTER, Acta Crystallogr. 12,884 7. E. M. GOPALAKRISHNA, Z. Kristallogr.
Sot.
study
99, 1384
(1959).
121,378
(1965).
Crystallogr. 14, 1009 (1961). 9. T. L. CHARLTON AND J. TROTTER,&& Crystallogr. 16,613 (1963). 10. J. TROTTER AND C. S. WILLISTON, Acta Crystallogr. 21,285 (1966). II. J.TROTTER, CQ~U~. J. Chem. 39,1638 (1961). 12. T. C. W. MAK AND J. TROTTER, Acta Crystallogr. l&68 (1965). 13. C. H. TOWNES AND B. P. DAILEY, J. Chem. Phys. 17,782 (1949); 20,35 (1952). 14. R. A. MARMO AND P. J. BRAY, J. Chem. Phys. 48,4833 (1968). 8. K. N. TRUEBLOOD,
15.
G. GOLDFISH,
AND J. DONAHUE,ACtU
16.
E. SCHEMPP AND P. J. BRAY, in “Physical Chemistry, An Advanced Vol. IV, Chap. 11, Academic Press, New York, 1970. (a) R. W. TAFT, JR., N. C. DENO, AND P. S. SKELL, Ann. Rev. Phys.
17.
JAFFE, Chem. Rev. 53,191 (1953). R. E. STARK, R. L. VOLD, AND R. R. VOLD,
Chem.
Phys.
20,337
(1977).
Treatise”
Chem.
(D.
Henderson,
9, 287 (1958);
Ed.),
(b) H. H.