NMRANALYSES AND PARAMETERS
991
Nuclear Magnetic Resonance Analyses and Parameters for Some Monohalosubstituted Fluorobenzenes
by J. E. Loemker,' J. M. Read, Jr.,2 and J. H. Goldstein Department of Chemistry, E m o r y Uniaersity, Atlanta, Georgia
3031.2 (Received August $1I 1967)
The nrnr spectra of a number of 0-,m-, and p-halo-substituted fluorobenzenes have been recorded and precise values of the nmr parameters have been determined by analysis. The H-F couplings have been compared with the corresponding couplings in fluorobenzene and, in addition] correlations between the H-F couplings and substituent electronegativity have been observed. These substituent effects vary in direction and magnitude, depending on the relative positions around the ring.
Introduction
larly evident in the para H-F couplings which have been observed to have either sign relative to the ortho and meta couplings. The latter couplings have been shown to be of the same ~ i g n . l ~ - ~ ~ At the present time, there have appeared very few complete analyses of the nmr spectra of monosubstituted fluorobenzenes. These have been limited to the para c o m p o ~ n d s , ~and, ~ ~ - even ~ ~ so, there are discrepancies in the parameters reported by various workers. In this article, we describe the results of a
Interest in substituted fluorobenzenes dates back to 1952 when Gutowsky, et a'.,3 reported that the influence of substituents on the el xtron distribution in benzene produces changes in the nuclear magnetic shielding of fluorine atoms in the molecules. Since this initial work, many substituted fluorobenzeries have been studied and their H-H, H-F, and F-F coupling constants have I n most cases, these compounds been have been polyfluorobenzeries and the data obtained have served to define approximate ranges for the H-F (1) NASA Predoctoral Trainee, 1966-1969. couplings. (2) Eastman Kodak Fellow, 1966-1967. I n a recent communication, Abraham, et aZ.,13 noted (3) H. S. Gutowsky, D. W. McCall, B. R . McGarvey, and L. H. that proton couplings in substituted benzenes are Meyer, J . A m e r . Chem. Soc., 74, 4809 (1952). relatively invariant to the nature of the ring substit(4) R. E. Richards and T. Schaefer, Proc. R o y . SOC. (London), uents, and from the limited fluorobenzene data availA246, 429 (1958). able, the H-F couplings appear t50 behave similarly. ( 5 ) S. 9. Dharmatti, M. M. Dhungra, G. Govil, and C. L. Khetrapal, Current Sei. (India), 31, 414 (1962). However, small systematic variations of the H-H coupling3 in benzene derivatives have been reported.l'%l4 (6) I. G. Aruldhas and P. Venkateswarlu, Mol. Phys., 7, 65 (1963). (7) B. Gestblom and S. Rodmar, Acta Chem. Scand., 18, 1767 As a result of a precise analysis of the benzene 13C-H (1964). satellites'j and analyses of a number of monosubstituted (8) G. W. Smith, J . Mol. Spectrosc., 12, 146 (1964). (9) W. G. Patterson and E. J. Wells, ibid., 14,101 (1964). b e n ~ e r i e s , 'it~ ~has ~ ~been possible to study these sub(10) G. Aruldhas, Proc. I n d i a n Acad. Sci., Sect. A , 63, 349 (1966). stituent coupling effects in some detail. Because of (11) B. Dischler, Z. Xaturforsch., 2Oa, 888 (1965). the observed short-range character of the substituent (12) M. Kimura, S. Matsuoka, S. Hattori, and K. Senda, J . Phya. effects, it appears that changes in the H-H couplings SOC.Jap., 14, 684 (1959). probably arise from inductive influences and are trans(13) R. J. Abraham, D. B. Macdonald and E. S. Pepper, Chem. Commun., 542 (1966). mitted primarily through the u electrons. illoreover, (14) P. F. Cox, J . Amer. Chem. SOC.,85, 380 (1963). it has been found that substituent effects on the H-H (15) J. M. Read, Jr., R. E. Mayo, and J. H. Goldstein, J . Mol. coupling parameters are approximately additive.l* Spectrosc., 22, 419 (1967). I n view of the results cited for H-H couplings in the (16) S. Castellano and C. Sun, J . Amer. Chem. Soc., 88, 4741 (1966). substituted benzenes, it appeared worthwhile also to (17) J. M. Read, Jr., and J. H. Goldstein, J . Mol. Spectrosc., 23, 179 (1967). examine the effects of substituents on H-F couplings (18) J. M. Read, Jr., R. W. Crecely, R. S. Butler, J. E. Loemker, in a series of substituted fluorobenzenes in order to and J. H. Goldstein, Tetrahedron Letters, in press. determine whether similar trends exist. The H-F (19) H. S. Gutowsky, C. H. Holm, A. Saika, and G. A. Williams, couplings are particularly interesting because the J . Amer. Chem. SOC.,79,4596 (1957). available data indicate that the variability of H-F (20) W. G. Patterson and H. Spedding, Can. J . Chem., 41, 2706 (1963). couplings is considerably greater than is the case for (21) D. F. Evans, Mol. Phys., 6, 179 (1963). H-H couplings. In addition, it appears that H-F (22) E. Lustig and P. Diehl, J . Chem. Phys., 44, 2974 (1966). couplings, quite unlike H-H couplings, are consider(23) C. Barbier, H. Faucher, D. Gagnaire, and A. Rousseau, J . ably influenced by long-range effects. This is particuChim. Phys., 63, 283 (1966). Volume 7.2, Number 3
March 1968
992 systematic investigation of H-F coupling constants which mere obtained by detailed computer analyses of the proton (and, in two cases, I9F)spectra of a series of 0-,m-, and p- halo-substituted fluorobenzenes. A total of ten halogen-substituted fluorobenzenes have been analyzed in this investigation, and the effects of the substituents on the nmr parameters at each position have been determined by comparison with the corresponding values in fl~or0benzene.l~Although the number of substituents employed is somewhat restricted, the results appear to determine adequately at least the effects produced by halogen substituents. In addition, the new parameters serve to supplement values previously reported for other dihalobenzenes.26
Experimental Section The ten monosubstituted fluorobenzenes were the commercially available materials, their purity being indicated by their respective nmr spectra. Most of the proton spectra were obtained at 60 MHz, using a Varian Model A60-A spectrometer at a probe temperature of 38". All samples were examined as solutions in tetramethylsilane (TXS) at a concentration of 10 mol % and were degassed before examination. Calibrations were performed by the usual side-band technique and the measured frequencies are the averages of three forward and three reverse sweeps. All of the spectral parameters reported here are based on these calibrated spectra. Two additional spectra were obtained as verification for the ortho-substituted fluorobenzenes: the fluorine spectrum of o-chlorofluorobenzeneobtained on a Varian RIodel HA 100 operating at 94.1 MHz and the proton spectrum of o-fluoroiodobenzene at 100 MHz obtained on a Jeolco 4H-100 nmr spectrometer.
Results and Calculations The compounds studied here are five-spin systems of the type ABCDX for the ortho- and meta-substituted compounds and AA'BB'X for the para-substituted compounds. The m-difluorobenzene is an ABB'CXX' six-spin system. The spectra were analyzed with the aid of Prospect-1, a least-squares computer program modified for an IBYI 1620 computer after LAOCOON II.26 The initial analyses were simplified somewhat by the availability of chemical shift and proton-proton coupling data for benzeneL5and by the use of additivity relationships found for the disubstituted benzenes. Substituent effects are calculated from the benzeneL5 and mon~halobenzene~~ parameters. The substituent effects on the chemical shifts were calculated in the manner previously described.2 7 - 3 0 The H-H coupling in a disubstituted benzene is calculated as the sum of the substituent effects from the two pertinent monosubstituted benzenes and the corresponding coupling in benzene.18 The initial estimates of the H-F coupling The Journal of Physical Chemistry
L. E. LOEMKER, J. M. READ,JR.,AND J. H. GOLDSTEIN
-415
-i/ /
-415
- 445
OBS
Figure 1. Additivity-calculated chemical shifts us. observed chemical shifts in the monosubstituted fluorobenzenes.
constants were obtained from reported values in similar molecules and were refined through trial and error calculations. Table I contains the observed chemical shifts along with the chemical shifts calculated from the additive substituent effects. These calculated shifts are plotted against the observed shifts in Figure 1, the solid line at a 45" angle to the axes corresponding to exact agreement of the two sets of values. As can be seen from this figure, with the exception of the proton ortho to the halogens in the 1,2-disubstituted benzenes, these calculated shifts are a good first approximation to the final values. The deviations found in the ortho proton shifts are comparable to those reported by Martin and Dailey30 for other disubstituted benzenes. Table I1 contains the observed H-H and H-F couplings along with the corresponding H-H couplings calculated from the additive substituent effects. These calculated and observed substituent effects on the H-H couplings are plotted in Figure 2, and again the correspondence is good enough to allow a close first approximation to the coupling parameters. In the case of o-chlorofluorobenzene, the parameters which reproduced the proton spectrum at 60 MHz were also adequate to predict the fluorine spectrum at 94.1 MHz, with the proton shifts at 60 MHz being converted to proton shifts at 100 MHz. The same type of correspondence also held for the proton spectrum of o(24) J . E. Loemker, J. M. Read, Jr., and J. H. Goldstein, Mol. Phys., 13, 433 (1967). (25) J. M. Read, Jr., R. W. Crecely, R. S. Butler, and J. H. Goldstein, Spectrochim. Acta, in press. (26) 5.Castellano and A. A. Bothner-by, J . Chem. Phys., 41, 3863 (1964). (27) J. B. Leane and R. E. Richards, Trans. Faraday SOC.,55, 707 (1959). (28) I. Yamaguchi and N. Hayakawa, Bull. Chem. Soc. Jap., 33, 1128 (1960). (29) P. Diehl, Helv. Chim. Acta, 44, 829 (1961). (30) J . S. Martin and B. P. Dailey, J . Chem. Phys., 39, 1722 (1963).
NMRANALYSES AND PARAMETERS
993
Table I : Observed and Calculated Proton Chemical Shifts in the Monosubstituted Fluorobenzenes" Compound
2
3
4
5
6
Fluorobenzene* 1,2-Bromofluorobenzene 1,2-Chlorofluorobenzene 1,2-Fluoroiodobenzene 1,3-Difluorobenzene 1,8-Bromofluorobenzene l,3-Chlorofluorobenzene lJ3-Fluoroiodobenzene lJ4-Bromofluorobenzene 1,4-Chlorofluorobenzene 1,4-Fluoroiodobenzene
-415.27
-430.96 -416.23 (-406.54) -418.27 (-410.17) -413.03 (-398.85)
-418.99 -426.00 (-425.66) -423.97 (-422.82) -428.01 (-428,23) -403.64 (-401.47) -411.33 (-409.97) -408.85 (-407.13) -413.01 (-412.54)
-430.96 -410.61 (-410.26) -415.00 (-413.89) -403.16 (-402.57) -428.03 (-429.13) - 421.90 (-422.23) -425.61 (-425.86) -412.87 (-414.54) -407.62 (-406.54) 409.92 (-410.17) -400.24 (-398.85)
-415.27 -444.23 (-441.14) -435.51 (-431.54) -457.11 ( -453.74) -403.64 (-401.47) -429.76 (-429.17) - 420.52 (-419.57) -441.72 (-441.77) - 438.67 (-441.14) -428.84 (-431.54) -449.49 (-453.74)
-400.49 ( - 397.75) -428.46 (-425.45) -418.78 (-415.85) -440.23 (-438.05) -438.67 (-441.14) -428.84 ( -431.54) -449.49 (-453.74)
-407.62 (-406.54) -409.92 (-410.17) -400.24 ( - 398.85)
-
" The nonfluorine substituent is assigned the number 1, and succeeding positions are numbered consecutively around the ring in the direction which gives the fluorine the lower position number, in Hz, a t 60 MHz, relative to TMS. Numbers in parentheses are the From ref 22, numbering begins with fluorine. calculated values.
Table I1 : Observed and Calculated H-H and H-F Coupling Constants in the Monosubstituted Fluorobenzenes" Fluorobenzene* 1,2-Bromofluorobenzene 1,2-Chlorofluorobenzene 1,2-Fluoroiodobenzene 1,3-Difluorobenzene 1,3-Bromofluorobenzene 1,3-Chlorofluorobenzene 1,3-Fluoroiodobenzene 1,4-Bromofluorobenzene lJ4-Chlorofluorobenzene 1,4-Fluoroiodobenzene
Jza
Jzr
JA
JZS
Jsr
J86
8.35 8.58
1.03 4.82
0.40 -0.63
2.58 6.82
9.06
4.72
-0.85
7.29
7.97
4.99
-0.24
6.29
9.14
2.45 (2.34) 2.51 (2.38) 2.45 (2.42) 2.50 (2.40) 4.83
7.50 8.25 (8.31) 8.26 (8.33) 8.24 (8.32) 8.22
1.76 1.48 (1.42) 1.46 (1.40) 1.42 (1.43) 6.45
8.31
5.99
-0.46
8.23
6.08
-0.59
8.42
5.85
-0.27
8.18
3.16 (3.01) 3.11 (2.99) 3.10 (3.02)
8.38 8.66 8.14 8.79 (8.89) 8.81 (8.97) 8.69 (8.75)
4.66 5.10
0.33 (0.20) 0.24 (0.26) 0.39 (0.27) 0.30 (0.24) 0.26 (0.26) 0.32 (0.27) 0.29 (0,24)
2.45 (2.34) 1.76 (1.77) 2.01 (1.93) 1.57 (1.57) 2.55 (2.52) 2.70 (2.68) 2.31 (2.32)
8.08 8.39
JSS
0.40 0.29 (0.26) 0.32 (0.27) 0.26 (0.24) -0.93
0.26 (0.26) 0.32 (0.27) 0.29 (0.24)
J4s
JlS
Jta
7.50 7.50 (7.38) 7.54 (7.46) 7.41 (7.39) 8.39 (8.25) 8.28 (8.31) 8.42 (8.39) 8.38 (8.32) 8.18
1.03 1.61 (1.54) 1.64 (1.58) 1.64 (1.56) 0.79 (0 75) 0.90 (0.79) 0.89 (0.83) 0.95 (0 I81) 4.83
8.08
4.66
8.39
5.10
8.35 8.02 (7.96) 8.01 (8.04) 7.89 (7.82) 8.39 (8.25) 8.06 (7.96) 8.02 (8,04) 7.88 (7.82) 8.79 (8.89) 8.81 (8.97) 8.69 (8.75)
I
' The nonfluorine substituent is assigned the iiumber 1, and succeeding positions are numbered consecutively around the ring in the direction which gives fluorine the lower position number, in Hz. Numbers in parentheses are the calculated values. From ref 22, numbering begins with fluorine.
fluoroiodobenzane at 60 MHz and that at 100 MHz. Such correspondence indicates that these parameter sets very probably constitute a unique solution.
It should be noted that, in these calculations, all of the H-H couplings were assumed to be of the same sign, taken to be positive, as is usually the practice in the Volume 79, Number 3 March 1968
994
vs. observed coupling substituent effects in the
L. E. LOEMKER, J. 11.READ,JR.,AND J. H. GOLDSTEIN
monosubstituted fluorobenzenes.
Figure 3. Experimental (top) and theoretical (bottom) proton spectra of o-chlorofluorobenzene a t 60 MHz.
case of benzene derivative^.^^^^^ Bak, et al.,33showed that in fluorobenzene the H-F and H-H couplings are of the same relative sign and hence the H-F couplings in fluorobenzene are taken to be positive. I n our recent analysis of fluorobenzene, we confirmed these relative signs, and, in addition, found that the para H-F coupling had a small (0.20 Hz) positive value.24 I n the present set of monosubstituted fluorobenzenes, we have found it necessary, for successful fitting of the spectra, to assume that the para H-F couplings in the ortho- and meta-disubstituted compounds are of opposite sign to that of the ortho and meta H-F couplings in the same molecules. This point is covered in some detail in the Discussion. I n general, the errors associated with the analyses are quite small. In no case was the root-mean-square error deviation between the experimental and calculated frequencies greater than 0.06 Hz, and individual deviations greater than 0.10 HZ occurred only in situations where there mere unresolved lines. The probable errors in the parameter sets are conservatively estimated t o be no greater than 0.04 Hz. Several examples of the observed and corresponding calculated spectra of the compounds analyzed are shown in Figures 3 and 4.
Figure 4. Experimental (top) and theoretical (bottom) fluorine spectra of o-chlorofluorobenzene a t 94.1 MHz.
Discussion Although considerable work has been done on the disubstituted benzenes, and, in particular, on the dihalosubstituted benzenes, relatively little attention has been directed to compounds containing one or two fluorine atoms. The symmetrical para-substituted fluorobenzenes have been rather extensively studied7~8~10-12 and, in the case of p-bromo- and p-chlorohave been reported fluorobenzeneJ by Gestblom and Rodmar.' In the case of the orthoThe Journal of Physical Chemistry
and meta-substituted fluorobenzenes, very little work has been done. Dharmatti, et aLj5studied ortho- and meta-substituted fluorobenzenes and attempted to correlate the para H-F couplings with the u- and xelectron densities in the ring. They concluded that the fluorine interaction with the para proton decreases steadily with an increase of the electronegativity of the substituent and that the effect is greater in the meta-substituted compounds with the substituent ortho to the para proton. However, their para H-F couplings for o-bromofluorobenzene (0.9 Hz), o-chlorofluorobenzene (1.0 Hz), m-bromofluorobenzene (1.9 Ha), and m-chlorofluorobenzene (1.8 Ha) are significantly different from those which are reported here. It should be noted that these previously reported para H-F couplings5 were obtained from a first-order analysis of the fluorine spectra. (31) R. W. Fessenden and J. s.Waugh, J . Chem. Phvs., 31,988 (1959). (32) C. N. Banwell, Mol. Phys., 4, 265 (1961). (33) B. Bak, J. N. Schoolery, and G. A. Williams, J. Mol. Spcctrosc., 2, 525 (1958).
NMRANALYSES AND PARAMETERS While this paper was being written, an analysis of the 100-MHz speictrum of m-difluorobenzene appeared in the literature.34 The H-H and H-F couplings reported in Macdonald’s paper agree quite well with those in Table 11, arid, in addition, he reports an F-F coupling of 6.57 Hz compared to our value of 6.50 Hz. Although Patterson and Wellse have reported H-H, H-F, and F-F couplings for the deceptively simple proton and fluorine spectra of p-difluorobenzene, complete analyses of the o- and p-difluorobenzenes have not been carried out. Many empirical correlations have been attempted in an effort to find some regularities governing the H-H couplings in substituted benzenes. We have carried out an analogous examination of the effect of substituents on the H-F couplings in this series of substituted fluorobenzenes. From the available data for HH and H-F couplings in substituted benzenes, the ranges for the H-F couplings appear to be considerably greater than those for the H-H couplings. Also, unlike the H-H couplings, these H-F couplings are influenced to a great extent by long-range effects. Substituent effects on the H-F couplings may be readily found by comparing the values listed in Table I1 with the corresponding fluorobenzene data.24 (The nomenclature used is explained in footnote a of Table 11.) For example, the ortho H-F couplings in the ortho-substituted compounds have a range of 7.979.06 Hz, compared to a fluorobenzene value of +8.90 Hz. The ortho H-F coupling, Jt3, in the meta-substituted compounds has a similar range, 8.14-9.14 Hz. It is worth noting that, in each of these two couplings, the substituent is ortho to either the fluorine or the proton in question and that the coupling increases with increasing substituent electronegativity. The ortho H-F coupling, J34, in the meta-substituted and para-substituted compounds decreases slightly with increasing substituent electronegativity. I n these two cases, neither of the nuclei in question is ortho to the substituent, and the couplings are all smaller than the corresponding fluorobenzene coupling. The ranges of these two ortho couplings are 8.23-8.42 Hz and 8.088.39 Hz for the meta- and para-substituted compounds, respectively. Hence one correlation which appears to be significant is shown in Figure 5 , which is a plot of these ortho H-F couplings vs. the Pauling electronegativity of the second substituent. Unlike the ortho H-H couplings, which show a definite increase with increasing electronegativity, these ortho H-F couplings show either an increase or a decrease depending on the position of the second substituent. Two of the meta H-F couplings exhibit consistent increases over the fluorobenzene value of +5.57 Ha, and two of the couplings exhibit decreases. These meta H-F couplings are plotted against the substituent electronegativity in Figure 6. The range of the values
995
Br
CI
I 3.0
1
F I 4.0
ELECTRONEOATlVlTY
Figure 5. Plot of JoHF us. the Pauling electronegativity of the second substituent: H, -Jsa in the ortho-substituted compounds; 0, J ~in s the meta-substituted compounds; -Jsa in the meta-substituted compounds; A, -Jg4 in the para-substituted compounds.
+,
-
ELECTRON KOATlVlTl
Figure 6. Plot of JmHF vs. the Pauling electronegativity of the second substituent: 0, -Jza in the ortho-substituted compounds; H, -$a5 in the meta-substituted compounds; A, -Jar in the ortho-substituted compounds; -Jz4 in the para-substituted compounds.
+,
for Jzs in the ortho-substituted compounds is 6.29-7.29 He, while JSs in the meta-substituted compounds (34)
D.B. Macdonald, Chem.
Commun., 686 (1967).
Volume 78, Number 3 March 1968
996
L. E. LOEMKER, J. 34. READ,JR.,A N D J. H. GOLDSTEIN meta H-F couplings in the ortho compounds and two ortho H-F couplings in the meta compounds. However, if in these two types of systems the sums are formed with either combination of ortho and meta couplings, or with the one coupling and an average of the two ortho or two meta couplings, the results partially agree with those of Hutton, et a1.36 Although it is true that, in general, increasing substituent electronegativity corresponds to increasing values of JoHFJmHF,the sum for the substituent, hydrogen, deviates considerably to the positive side of any of the possible curves. The reason for this deviation can be seen in Figure 5 . For the ortho and meta compounds, each of the plots of the ortho H-F couplings vs. electronegativity also lies considerably below the point for hydrogen. I n the ortho- and meta-substituted fluorobenzenes studied here, the para H-F couplings, although relatively small in magnitude, were found to have a sign opposite to that of the ortho and meta H-F couplings. In order to verify this result, several additional calculations were carried out for the fluorine spectrum of o-chlorofluorobenzene. These calculated spectra are shown in Figure 8. The observed spectrum and the calculated spectrum with J p H F= -0.85 are shown in Figure SA and B. Figure 8C is the calculated spectrum in which all of the H-F couplings are of the same sign, here taken to be positive. As can be readily seen from the figure, the latter calculated spectrum does not correspond to the observed spectrum. The final
+
LH
I
1
BI
c1 I 3.0
F I
I
4.0
ELECTRONEOATIVITY
Figure 7. Plot of JPHF us. the Pauling electronegativity of the second substituent: 0, -J26 in the ortho-substituted compounds; W, -Jle in the meta-substituted compounds.
assumes values in the range 5.85-6.45 Hz. The other two meta H-F couplings, J24 in the ortho-substituted compounds and J24 in the para-substituted compounds, have ranges of 4.72-4.99 Hz and 4.66-5.10 Hz, respectively. I n these cases, it is difficult to perceive any well-defined relation between the substituent position and the corresponding increase or decrease in the parameter. However, it will be noted that in the two cases where the coupling increases, both nuclei are either ortho to the substituent or meia to the substituent. The para H-F couplings in both the ortho- and metasubstituted compounds decrease from the fluorobenzene value of +0.20 Hz. These para H-F couplings are plotted against the substituent electronegativity in Figure 7. I n the ortho-substituted compounds, J Z 6has a range from -0.63 to -0.24 Hz, and in the metasubstituted compounds, J36 has a range from -0.93 to -0.27 Hz. As mentioned previously, Dharmatti, et discussed the effect of the substituent position on the magnitude of the para H-F couplings. Our data are essentially in agreement with their conclusion that the relatively larger effect is caused by the substituent ortho to the proton. While this manuscript was in preparation, there appeared a publication by Hutton, Richardson, and SchaefeF in which these authors demonstrated an empirical relationship between the sum JoHF J m H F and the electronegativity of substituents ortho, meta, and para to fluorine in the substituted fluorobenzenes. We have prepared similar plots (not shown here) using the data in Table I1 and the fluorobenzene data. In the case of the para-substituted compounds, the result is a smooth curve, which supports a previous conclusion that an increase in substituent electronegativity corresponds to a reduction in JoHF JmHF.35For the oriho- and meta-substituted fluorobenzenes, the corresponding sums cannot be obtained, since there are two
I
The Journal of Physical Chemistry
B
C
I
I
I l#lllll I I I
+
+
lllll
n--r
D
Figure 8. A. Observed fluorine spectrum of o-chlorofluorobenzene a t 94.1 MHz. B. Corresponding calculated spectrum with JPHF= -0.85 Ha. C. Calculated spectrum with all JPHF’staken as positive. D. Calculated spectrum with the relative signs of the P F ’ s the same as in spectrum B, but with JPHF = +0.85 Ha. (35) H. M. Hutton, B. Richardson, and T. Schaefer, Can. J. Chem., 45, 1795 (1965).
PHOTOCHROMISM calculated spectrum in this Figure (8D) is one in which the relative signs were taken to be the same for the H-F couplings, as in Figure 8B, but with J p H F = +OB5 Hz. The effect of this choice was merely to invert the entire spectrum. Hence, it seems reasonable to conclude that the pura H-F couplings are of opposite sign from that of the other H-F couplings and that the ortho and metu H-F couplings have the same sign relative to the H-H couplings. I n summary, the H-F coupling constants reported here exhibit rather wide ranges of values which are consistent with the ranges previously reported for other derivatives. Unlike the corresponding H-H couplings in the monosubstituted benzenes, all of the H-F couplings in the fluorobenzenes are significantly affected by the substituent. The correlations found between the H-F couplings and substituent electronegativity are interesting, but not readily explicable. It is possible that these couplings may b e influenced not only by inductive factors, which seem largely responsible for variations in aromatic H-H couplings, but also
997 by some combination of mesomeric and geometric variations. Previous theoretical studies have indicated that the interpretation of H-F couplings presents, in general, a more difficult task than is the case for H-H coupling^,^^^^^ and there are further complications in aromatic systems. The present status of this problem indicates a need for additional investigations, both theoretical and experimental. Acknowledgments. This research was supported in part by a grant from the National Institutes of Health. The authors also wish to express their appreciation to R9r. Bill Jankowski, Analytical Instruments Application Laboratory, Varian Associates, Pittsburgh, Pa., for the fluorine spectra of o-chlorofluorobenzene and m-difluorobenzene at 94.1 1IHz. The proton spectrum of o-fluoroiodobenzene at 100 1SHz was provided by Jeolco, Medford, Mass. (36) J. A. Pople, Mol. Phys., 1, 216 (1958). (37) G. A. Williams and H. S . Gutowsky, J. Chem. Phys., 30, 717 (1959).
Photochromism: Spectroscopy and Photochemistry of Pyran and Thiopyran Derivatives by Ralph S. Becker and Jaroslav Kolc Department of Chemistry, University of Houston, Houston, Texaa '7'7004 (Received September. 1, 1967)
Several pyran and thiopyran derivatives, including an indolinospirobenzothiopyran, exhibit photochromic behavior. It is possible to sensitize coloration of the latter compound in a rigid matrix. The substitution of sulfur for oxygen causes dramatic red shifts in the absorption bands of the colored but not of the colorless compound. Structures for the colored products have been assigned. The photochemical process appears t o be highly efficient.
Introduction As part of our continuing investigation to evaluate the various spectroscopic parameters of molecules, energy transfer, and the importance of these in photochemistry, we have investigated several new molecular systems. One particular aspect worthy of clarification is the site of photochemical activity in the chromenes and indolinospirobenzopyrans. Another important area is the evaluation of the effects of substitution of sulfur for oxygen. Of particular concern are the spectroscopic properties of both the uncolored and colored forms, as well as the photochemical behavior of the uncolored form.
We wish to report reversible photochemical behavior for four new molecular ring systems: 2H-pyran, 2H-thiopyranJ 2H-thiochromeneJ and indolinospirobenzothiopyran.
Experimental Section All experiments were carried out in 3-methylpentane a t 77"K, unless otherwise noted. The absorption spectra were determined by means of a Cary Model 1.5 recording spectrophotometer with quartz cells 2 mm and 10 mm in path length. Concentrations were approximately fM. For production of the photocolored forms, a 1-kW Hg-Xe lamp with a Corning Volume 7% Number 3 March 1968
