1830
A. A. BOTHNER-BY AND R. H. Cox
an aqueous reference solution and an AgC1-Ag electrode. To change the reference half-cell to that stated in eq 15, the aqueous reference solution was replaced by dry acetic acid containing 0.003 F of anilinium chloride, 0.0002 F of aniline, and saturated with AgC1. The “combination electrode,” so modified, was stored in acetic acid for at least 12 hr before the start of a series of measurements. To begin the series, the reference solution in acetic acid was replaced by a fresh solution of similar composition, and 5 ml of solution X* (eq 15) was placed in the jacketed cell. The Teflon stopper, combination electrode, and microburet were mounted in place, the flow of water at 26.2” was
begun, and the cell was allowed 30 min to come to equilibrium before its emf was recorded. A series of solutions X (eq 15) was then prepared directly in the cell by adding from the microburet successive portions of 0.003 F m-chloroaniline or 0.003 F p-toluenesulfonic acid, respectively. After each addition the new solution was stirred for 1 min and the emf was read after waiting for another minute. Emf was read conveniently on the millivolt scale of the pH meter, which was calibrated frequently by comparison with a standard cell. Each series of measurements was repeated at least once, and w was reproducible to about 2%.
The Estimation of Vicinal Phosphorus-31-Hydrogen and Nitrogen-14-Hydrogen Spin-Spin Coupling Constants in Single Conformers by A. A. Bothner-By and R. H. Cox Mellon Institute, Carnegie-Mellon University, Pittsburgh, Pennsylvania 16229
(Received December 18, 1968)
The magnitudes of the various vicinal constants in a disubstituted ethane, XCH2CHzY, vary linearly with each other, provided the changes are caused only by changes in rotamer population. The phenomenon is illustrated in detail with the example of C1CH2CH2PSC12. Using appropriate values for the a J H H constants, and 3JPRL7ans for this compound are obtained. Similar treatments are presented briefly estimates of VPHBauche for the compounds (CH&CCH&H2NfC and H,C=CHCH,N T C ; the analysis of the spectrum of vinyl isocyanide is given. The values obtained from these measurements, combined with those obtained from the ~ a~ Jcoupling ~ ~constants show a dependence on dihedral angle literature, support the hypothesis that a Jand similar to that of 3 J H B .
Introduction Evidence’ is accumulating that vicinal couplings between a hydrogen and an element of the first or second row (e.g., H-CL3,H-N, H-F) depend on the H-C-C-X dihedral angle in a way similar to the well-studied2 3J”. As a contribution to the exploration of the factors affecting the magnitude of 3JPH and 3J”, we wish to and BJXHtrans in ethanic present some values of 3JXHgauche fragments, obtained by a new method, as well as some other miscellaneous observations. The stable rotamers of a 1,Zdisubstituted ethane may be represented as in Figure 1. If the population of the trans rotamer be designated by 1 - 2 p , p will represent the population of each of the two (mirror image) gauche rotamers, which will be equally populated in a nondissymetric solvent. Values of 0.0 to 0.5 may be assumed by p . Under conditions of rapid interconversion of the rotamers, the coupling constants obtained by an analysis of the magnetic resonance spectrum (spectra) will be The Journal of Physical Chemistry
the population-weighted averages of the coupling constants in the individual rotamers. Thus
3J”
=
(1 - 2p)3J”I
+ p3J”11 + p3JHHm
(1) ( C 9 G. J. Karabatsos, C. E. Orzech, Jr., and N. Hsi, J . Amer. Chem. SOC.,88, 1817 (1966); G. J. Karabatsos, N. Hsi, and C. E. Orzech, Jr., Tetrahedron Lett., 4639 (1966); C. Benezra and G. Ourisson, Bull. Soc. Chim. Fr., 1825 (1966); (“4 and N16) Y. Terui, K. Aono, and K. Tori, J . Amer. Chem. Soc., 90, 1069 (1968); A. K. Bose and I. Kugajevsky, Tetrahedron, 23, 1489 (1967); (F’g) C. N. Banwell, N. Sheppard, and J. J. Turner, Spectrochim. Acta, 16, 794 (1960); V. S. Watts and J. H. Goldstein, J . Chem. Phys., 42, 228 (1965); K. L. Williamson, Y. F. Li, F. H. Hall, and 9. Swager, J. Amer. Chem. SOC.,88, 5678 (1966); G. Aranda, J. Julian, and J. A. Martin, Bull. Soe. Chim. Fr., 2850 (1966); (SizQ)9. 5. Danyluk, J . Amer. Chem. SOC.,87, 2300 (1965); (P39 C. Benezra and G. Ourisson, Bull. Soc, Chim. Fr., 1825 (1966); M. P. Williamson, S. M. Castellano, and C. E. Griffin, J. Phys. Chem., 72, 175 (1968);
W. A. Anderson, R. Freeman, and C. A. Reilly, J . Chem. Phys., 39, 1518 (1963); J. E. Lancaster, Spectrochim. Acta, 23A, 1449 (1967); G.L. Kenyon and F. H. Westheimer, J . Amer. Chem. Soc., 88, 3557 (1966); M. L. Maddox, Thesis, University of California at Los Angeles, 1965. (2) A. A. Bothner-By in “Advances in Magnetic Resonance,” Vol. I, J. S. Waugh, Ed., Academic Press, Inc., New York, N. Y . , 1965, pp 195-316.
1831
ESTIMATION OF VICINAL COUPLING CONSTAKTS
or
H
Y
+
= aJ"I
3J"
+ ~J"III- 2J"r)
p(3J"II
(1)
where 3J" is the observed coupling, 3J"~is the value of 3J" in rotamer I, etc. Supposing X or Y or both to be magnetic nuclei giving rise to observable couplings, equations of identical form may be derived for the remaining vicinal couplings =
3~"'
+
+ p(3~"'11
3~"'~
1-2p
P
P
I
l I
E t
Figure 1. Stable conformers and populations of a 1,2-disubstituted ethane.
-
3 ~ " ' ~ ~ ~23JHH'I)
(2) 3JHX 3
~
3~~~
=
3JHXI
= ~
3'
~
+ p ( 3 J H X+~ 3~ J H -X2~3 J~H X~~ )(3) + + (4)
=
3
~
+
~~ (
3~
~
1~ 8 ~ ~ 1 ~ 1~ 1 12 13
+
~
~
~
1
)
-
p~ ( a ~ ~~ ' I 11 3 ~ ~ ~ 1 12 13 ~ ~ ~ (5) 1 )
Symmetry may result in some simplification of these equations: for example, 3 J X y I I = 3 J X Y and ~ ~ ~if , X = Y, 3J"'I~ = ~J"'III. Following the example of Abraham and C a ~ a l l iwe , ~ may take any two of these equations, for example (1) and (4)) and eliminate p between them to obtain =
3J"
SaJHY+ I
(6)
where
+
+
J!
S = ( 3 J " ~ ~3 J " ~-~~J"I)/(~J~'II ~
-
3 ~ ~ ~ 1 12 13
and
I
=
3JHHI
-
~
~
~
S3JHYr
Thus, if 3JHH is plotted against 3JHP, a straight line should be obtained, with slope S and intercept I. This line will pass through the points appropriate for p = 0.0 and p = 0.5, and from these points, if they can be located, the ordinates and abscissas will give pairs of characteristic for rotamer I and values of 3J" and aJHY for an equal mixture of rotamers I1 and 111; Figure 2 illustrates these relations. Several methods to locate the points corresponding to p = 0 or 0.5 may be employed. The rotamer populations corresponding to the observed points may be measured by an independent method, such as ir or dipole moment measurements; the populations may also be measured by theoretical treatments of the solvent3p4or temperature6 dependence of the coupling constants; finally, knowledge of the values of 3 J " derived from the large number of recorded observations2 on analogous internally rotating or rigid systems may be used to 3J"~~ etc., ~, choose appropriate values of 3J"~,3J"~~, and to fix the points directly. I n this paper the last method is applied.
Experimental Section Chloroethylthiophosphoryl Dichloride. This material was most generously supplied by Professor C. E. Griffin. The sample examined by nmr was at 15 mol yo concentration in bromoform solution.
J~
+(J:+J~)
Figure 2. Plot of one coupling constant us. another when both ) depend linearly on conformer population.
1
3,S-Dimethylbutyl Isocyanide. The procedure of Jackson and McKusicke for the preparation of ethyl isocyanide was followed, except that an equivalent amount of 3,3-dimethylbutyl iodide was substituted for ethyl iodide. The yield was 30% of a colorless liquid, bp 151-155'. The sample was examined in 15 mol % solution in perdeuteriocyclohexane, with an added trace of tetramethylsil ane. Allyl Isocyanide. The procedure of Jackson and McKusick6 was used, except that allyl iodide6 was substituted for ethyl iodide; yield, 40% of colorless material, bp 83-85'. The sample examined was undiluted except for an added trace of tetramethylsilane. V i n y l Isocyanide. The procedure of Matteson and Bailey' was employed. The yield of vinyl isocyanide, bp 45-47', was 42% (lit.' 47%). The sample was examined at 15% concentration in these solvents : ethanol, carbon tetrachloride, dimethyl sulfoxide. Nmr Spectra. Proton magnetic resonance spectra were recorded with a Varian A-60 spectrometer, equipped with a variable temperature probe, or with a (3) R. J. Abraham and L. Cavalli, Mol. Phys., 11, 471 (1966). (4) R. J. Abraham and M. A. Cooper, J . Chem. Soc., B , 202 (1967). (6) G. Govil and H. J. Bernstein, J . Chem. Phys., 48, 285 (1968). (6) H. L. Jackson and B. C. McKusick, "Organic Syntheses," Vol. I V , John Wiley & Sons, Inc., New York, N. Y., 1963, p 438. (7) D. S. Matteson and R. A. Bailey, Chem. Ind. (London), 191 (1967). Volume 73, Number 6
June 1069
1832
A. A. BOTHNER-BY AND R. H. Cox
Figure 3. Experimental and calculated 6O-h/IHz proton nmr spectrum of p-chloroethylthiophosphoryl dichloride.
Varian HA-100 similarly equipped. All spectra were recorded at least four times, twice with sweep toward higher fields, twice with sweep toward lower fields, The spectra were calibrated by means of audio side bands, generated by field modulation using a General radiofrequency synthesizer. The samples were degassed and sealed in 5-mm thin-wall Pyrex sample tubes. Analyses of the spectra were performed using the L A O C N ~p r ~ g r a m . ~ ' ~
Results Chloroethylthiophosphoryl Dichloride. The parameters obtained by the analysis of the proton spectra of the bromoform solution of chloroethylthiophosphoryl dichloride at several temperatures are presented in Table I. A comparison of the room temperature ob~~
Table I : Nuclear Magnetic Resonance Parameters (3)H H(2)
1 I
PSClZ
(4)H H(1) parameter
_--_____---
___- -
Temp, O C
30
48
73
100
120
335.64 396.38 -13.38 5.37 10.28 -10.09 -13.22 14.18
335.01 395.92 -13.32 5.43 10.16 -10.09 -13.24 14.54
334.15 395.19 -13.25 5.51 9.96 -9.95 -13.25 15.12
333.36 394.70 -13.21 5.56 9.78 -9.92 -13.27 15.50
332,85 394.24 -13.10 5.63 9.67 -9.81 -13.34 15.99
I n Hertz from tetramethylsilane.
b
In Hertz.
served and best calculated spectrum is given in Figure 3. The root-mean-square error in fitting the frequency of observed lines was 0.05 Hz or less. 3J" and 3J"t were assigned on the basis that the approximation of a single gauche and trans coupling constant yields the relation 23J" + 3J"' 2J gauche JZr5m
+
The Journal of Physical Chemistry
3J" = -0.34g3JPH
+ 15.23
and 3J"'
=
+0.1423JPH
+ 3.36
Figure 4 shows a plot of these lines. Reasonable estimates2 for the H,H coupling constants in the conformers are 3J"~= 13.0 Hz, 3JHH'~ = 4.2 Hz, 0.5(3J"~ 3 J " ~ ~ ~=) 6.8 Hz, and 0 . 5 ( 3 J H H ' ~ ~ 3 J " ' ~ ~=~4.2 ) Hz. These are consistent among themselves in defining the loci of the p = 0 and p = 0.5 lines. Furthermore, the point at which the two lines intersect, i.e., where 3JHH - 3J"', should approximately define the locus of p = 0.33, and this point lies two-thirds of the distance from the line p = 0 to the line p = 0.5 within experimental error. These assignments allow the values
+
+
of Chloroethylthiophosphoryl Dichloride
1 I
irrespective of conformer population. l o Twice JI3added to JU gives an approximately constant sum (20.922 to 21.031 Hz) of the expected rnagnitude,l' while the reverse assignment gives a sum which varies by more than 1 Hz over the temperature range covered and is unreasonably large. Thus 3JHH corresponds to the small coupling JI3and the structural formula at the head of Table I is a Fischer projection showing the numbering of the protons correctly. Least-squares treatment of the data gives the relations
3JPH~ = 6.3 HZ and
+
0 . 5 ( 3 J P H ~ ~'JPH1ll) = 31.4 HZ to be read off directly. Making the approximation of a single gauche and trans coupling constant for 3JPH, this yields a gauche-coupling constant of 6.3 Hz and a transcoupling constant of 56.5 Hz. (8) Available through the Quantum Chemistry Program Exchange, Chemistry Department, Indiana University, Bloomington, Ind. 47401. (9) 8. M. Castellano and A. A. Bothner-By, J . Chem. Phys., 41,3863 (1964). (10) D.Jung and A. A. Bothner-By, J. Amer. Chem. SOC.,86, 4025 (1964). (11) R.J. Abraham and K. G. R. Pachler, Mol. Phys., 7, 166 (1963).
1833
ESTIMATION OF VICINALCOUPLING CONSTANTS may still be performed as above, yielding
3J" = -l.l3(J}
+ 10.28
+
where (J) = 0.5(3J" 3J"'). For conformer I, (J) is estimated t o be 8,5 He, yielding JNHBauche = 0.7 Hz; for conformers I1 and I11 (J) is estimated t o be = 6.9 Hs. 5.75, giving JHHtrans A l l y l Isocyanide. Results of the measurements on allyl isocyanide are presented in Table 111. The three conformers in this case may be represented as in Figure 5. The 3J proton-proton and nitrogen-proton cou-
\
k
4
------
*o
!i \
d
1 3 P H 3 P H JII+ ,J
3JPH
. I
10
20
Table 111: Nmr Parameters of Allyl Isocyanide (Neat) at Several Temperatures (3)H
I
\
30
3 ~ P H(HZ)
Figure 4. Plot of aJHH and 3J"' us. aJHP for 0-chloroethylthiophosphoryldichloride. Vla
The probable errors to be attached t o these values are subject to discussion, the main contribution being from the estimation of the H,H coupling constants for the individual conformers. Proton-proton coupling constants in unstrained systems may be predicted with a probable error of about 0.2 Hs, which makes a probable error of f2 He a conservative value for the phosphorus proton coupling constants. $,$-Dimethylbutyl Isocyanide. Table I1 gives the
Table I1 : Temperature-Variable Coupling Constants in 3,3-Dimethylbutyl Isocyanide
r
CHa- -CHz-CHz-"N=C
93 68 51 35 10 - 12 a
(J) = 0.5(aJ"
aJ N H
1.694b 1.669 1.607 1.476 1.374 1.275
+ aJ"').
b
va v4
V6 Jiab J14
JIG Jl6
J84
Jaa J4a JGB
T =
T =
T =
T =
- 350
14'
25'
62'
244.20 316.10 323.63 347.97 -1.98 -2.12 4.33 -1.97 0.86 10.33 16.88 2.67
242.68 315.72 323.31 347,98 -1.87 -1.94 4.56 -1.97 0.84 10.35 16.80 2.43
242,050 315. 67c 323.25< 348. 04c -1.84 -2.02 4.64 -2.15 0.89 10.37 16.88 2.44
241.18 315.26 322.92 347,88 -1.82 -1.94 4.74 -2.03 0.82 10.35 16.85 2.18
a In Hertz from tetramethylsilane ( y o = 60 MHz). In Hertz. Spectrum obtained at 100 MHz; chemical shifts multiplied by 0.6 to convert into 60-1\IIHz basis.
Table IV : Nmr Parameters of Vinyl Isocyanide in Several Solvents
&Ha T ,O C
c=c
(3)H
(J)"
7.609 7.660 7.716 7.781 7.910 8.000
In Hertz.
(2)H Parameter v1b
v2 Va Jizc Jl3
nmr parameters obtained for 3,3-dimethylbutyl isocyanide in perdeuteriomethylcyclohexane solution over the temperature range -12 to +93". I n this case deceptive simplicity of the spectrum prevented the separate determination of 3J" and 3J"'; only the sum could be obtained. However, the treatment of the data
J2a
J1N JZ N
JaN
\ c=c/ / \
4N C
H(1)
_________-
__________solvents
Ethanol
ccI4
364.13 323.61 335.31 8.49 15.52 -0.66 2.31 6.28 3.20
356.03 320.97 335.12 8.50 15.59 -0.58 2.25 6.19 3.04
DMSO
374.53 329.25 341.10 8.57 15.55 -0.52 2.33 6.31 3.22
a 10 vol yo solutions. b In Hertz at 60 MHz from tetramethylsilane. c I n Hertz.
Volume 78, Number 6 June 1069
1834
A. A. BOTHNER-BY AND R. H. Cox
/-
:
c;
c1
CS
I I I I I
I I
Figure 5 . Expected conformers of allyl isocyanide.
I I
I
_I
o/ 0
I
/
//
I
0"
Ll2-U-
O0
1
I
1
I
I
goo
+
11
Figure 7. Approximate dependence of 3 P N (for 14N)on dihedral angle: 0, data of Terui, et u L ; ~ 0, d a h from present work.
90'
+
Figure 6. Approximate dependence of VPHon dihedral angle: 0, data of Benezra and Ourisson;' 0 , data from present work.
pling constants across the single bond should vary linearly with each other, the least-squares line obtained from the data, being
3J" = -1.15'J"
+ 7.70
For conformer C, the predicted12 value of J" is 2.4 = 4.9 Hz. For an equal mixture Hz, yielding 3J"lrans of the C1conformers, 3J" will be close to 6.5 Hz, giving 3J"gauche = $0.2 Hz. Vinyl Isocyanide. The spectrum of vinyl isocyanide in several solvents has been ana1y~ed.l~The spincoupling parameters obtained are given in Table IV. The relative signs of the proton-proton coupling constants and the relative signs of the N'eproton coupling constants were determined by spin-tickling experiments, confirming the results of Ohtsuri and Tori14 on trimethylvinylammonium bromide.
The Journal of Physical Chemistry
14N-H and 31P-H Coupling Constants. A plot if 3JPH vs. dihedral angle for the case reported here and other cases in the literature1 is given in Figure 6. All cases reported thus far are consistent with a dependence on dihedral angle similar to that of 3J". The plot includes points for 31Pand H bound t o trigonal as well as to tetrahedral carbon, and, as in the case of the H-H couplings, this change in hybridization does not produce a great effect in strain-free compounds. A plot of 3J" vs. dihedral angle has a similar appearance (Figure 7), and further support is given to the suggestion of Terui, et aL,l that these couplings also conform to a Karplustype curve. Acknowledgment. This work was supported by a grant from the National Institutes of Health, FR00292. (12) A. A. Bothner-By, S. M. Castellano, S. J. Ebersole, and H. Gunther, J . Amer. Chem. Soc., 88, 2466 (1966). (13) After this work was complete, an analysis of the vinyl isocyanide spectrum was reported by D. S. Matteson and R. A. Bailey, ibid.,90, 3761 (1968). Our results agree closely with theirs. (14) M.Ohtsuri and K. Tori, Chem. Commun., 750 (1966).
