Solvent and Temperature Effects on One-Bond Spin-Spin Coupling Constants

Jennifer C. J. Barnat and Michael J. T. Robinson* Dyson Perrins Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QY, UK

Intrinsic solvent and temperatme effects on 'J(CX) (X = H, C, F) have implications for users of one-bond couplings in conformational analysis.

INTRODUCTION

In the course of studies of the application of 'J(CC), 'J(CH) and 'J(CF) in conformational analysis, unex- pectedly large intrinsic solvent and temperature effects on the couplings were observed. These are potentially important sources of error in conformational analysis whenever changes in Jobs, where Jobs = C n,Ji and Ji is the value of the coupling constant for the ith con- former present in molar fraction n, (c n, = l), are used to infer changes in conformational equilibria.

~

RESULTS AND DISCUSSION

Solvent effects' are demonstrated using l3CH3I (Table 1). Recently: dielectric constants have been found to affect 'J(CH) much more than does temperature, but our data (Table 1) correlate best with the electric dipole moment of the solvent molecules. There is also

Table 1. Solvent effects on 'J(CH) for "3Jiodomethane (1 moI dn-") (see Experimental for estimates of errors)

Dipole Dielectric momentb

Solvent constant- debye 'J(CH)/Hz

CD2CI, (CD,),CO CD,OD

(CH,),Si CF,CO,H

HCONMe,

'6'6

C6H12

8.75 1.60 20 2.88 33.6 1.70" 2.26 0 1.91 0

2.02 0 39.5 2.28"

36.7 3.82

151.33 151.46 151.12 151.12 151.21 151.26 151.3Id 151 .5gd

a Isotopically normal, 308 K.'= Isotopically normal, gas phase? Formation of hydrogen bonded polymers

in the liquid probably leads to a lower effec- tive dipole moment in the liquid for any solute that does not form hydrogen bonds.

Measured using concentrated sol~tions.~

* Author to whom correspondence should be addressed. T Present address: University Chemical Laboratory, Lensfield Road, Cambridge CB2 lEW, UK.

a fair correlation between effects of hydrogen bonding on 'J(CH) for chloroform' and for iodomethane, sug- gesting very weak hydrogen bonding6 between iodomethane and, for example, acetone.

Temperature effects are given in Table 2. The in- trinsic temperature dependence of coupling constants has long been known for 'J(€€H)' and 3J(FF),10 but not for 1J.1171z The only theoretical study of tempera- ture dependence appears to be one of 'J(HX) in dihydrogen and its i~otopomers,'~ with 'J(HX) pre- dicted to increase with temperature because of in- creased rotational motion. This does not rationalize our results as 'J(CH) and 'J(CC) change in opposite senses with temperature. Factors that might affect the temperature dependence of couplings are" (a) tor- sions, (b) other vibrations and (c) intermolecular in-

Table 2. Temperature dependence of '$(-) in some simple compounds in CD,Q (see Experi- mental for estimates of errors)

Concentration1 AJ/ATb/

13CH31 H 1 151.29 -0.004 4 151.70 -0.005

13CH,13CN 2.5 136.10 -0.001 13CH,13CH,F F 1.6 160.20 -0.0188

Compound X rnoldm-3 'J(CX)/Hza KHz-'

13CH,Br

'3CH313CH,I C 1 35.81 +0.0012 Neat 35.87 +0.0011

'3CH,'3CH,CI 1.5 35.17 +0.0011 l3CH3l3CH,F 1.6 38.16 +0.0008 13CH,13CN 2.5 56.& +0.0028

0.5 57.63 +0.0025 13CH,'3C0,Me 0.5" 59.33 +0.0018

'3CH,13CH,Br 1.5 35.97 +0.0012

'3CH,'3C0,Na 0.5" 52.30 -0.0003 13CH,13C0,Rd 0.05e 60.08 -

0.05' 60.03 - 0.05' 59.86 - 0.05h 59.80 - 0.05" 59.78 -

a At 298 K; allowing for differences in solvent, etc., these values agree with less precise published values for X=H7 and X=C?

Normally over the range 200-300 K. Methanol-d, as solvent. R = p-bromophenacyl. Benzene-d6 as solvent.

' Acetone-d, as solvent. a Acetonitrile-d, as solvent.

Chloroform-d as solvent.

CCC-0749- 1581/85/0023-0192$01.00

192 MAGNETIC RESONANCE IN CHEMISTRY, VOL. 23, NO. 3, 1985 0 Wiley Heyden Ltd, 1985

SOLVENT AND TEMPERATURE EFFECTS ON 'J(CX) (X = H, C, F)

teractions. 'J(CC) in ethane is predicted to be greater for the eclipsed form than for the staggered form,14 but there is no clear correlation with three-fold torsion barriers (Table 3: compare EtF and EtI). There is a fair correlation between AJ/AT and kB (Table 3) for haloethanes but not for sodium acetate (AJIAT = 0). There is no direct correlation between stretching force constants21 and AJIAT. The last five entries in Table 2 show solvent effects on 'J(CC) for p-bromophenacyl [13CJacetate. There is a negative correlation with the hydrogen bonding donor strength of the solvent con- sistent with the temperature dependence of 'J(CC) for methyl [13C2]acetate in methanol, in which hydrogen bonding should decrease with increase of temperature.

Table3. Some physical properties of com-

Compound D' kmb YC VSd

CH,CH,F 1.94 1.22 (415) 13.83 CH,CH,CI 2.05 0.96 - 15.42 CH,CH,Br 2.03 0.90 - 15.41 CH,CH,I 1.91 0.82 - 13.47

CH,CO,Me 1.72 - 42916 1.25 CH,CO,Na - - 4711' 0

a D = Dipole moment, gas phase, in debyes., bkg=Force constant for CCX bend in mdyn A rad-,." u = Low-frequency frequency (in plane bend-

ing) in cm-'. The value for fluoroethane was tentatively assigned from the, literature" cited by Meyer and Allinger18 but values for the other haloethanes were not identified.

pounds in Table 2.

CH,CN 3.92 - 38015 o

V,=Three-fold torsion barrier in kJ mol-l.2'

EXPERIMENTAL

Measurements were made using isotopically substi- tuted compounds. These were usually prepared using 13CH30H and Ba13C03 (enriched to at least 90 atom- YO in 13C), supplied by BOC Prochem. Doubly isotopi- cally substituted compounds, therefore, had an isotopic abundance of approximately 80 atom-% of 13C. 13CH31 and 13CH3Br (60 atom-%) were obtained directly from BOC Prochem.

Coupling constants were measured using a Bruker WH90 FT NMR spectrometer operating at 22.63 M H Z , with a Nicolet B-NC 12 computer. Sweep widths of 50-100 Hz ['J(CC)] or 500-625 Hz [lJ(CH), 'J(CF)] were used. FIDS were accumulated in 1K ['J(CC)] or 2K ['J(CH), 'J(CF)] memory addresses. After line broadening the data memory was increased to 8K (zero filling) before FT and approximate phas- ing. Using a program that assumed an 8K data table, data were transferred via a 16-bit parallel interface to a Hewlett-Packard 9825B calculator for least-squares curve fitting to Lorentzian-shaped lines with automatic refinement of the phasing.22 This gave standard errors of *0.002 to +0.010 Hz for individual measurements of 'J(CC), with similar r.m.s. residuals for the linear correlation between 'J(CC) and T ; these errors have been rounded up to * O . O l H z for 'J(CC). Standard errors for 'J(CH) and 'J(CF) have similarly been rounded up to +O.O4Hz.

Acknowledgements

We thank the SERC for a Studentship and Research Assistantship (to J.C.J.B.) and for a grant to the laboratory towards the cost of the Bruker WH90 spectrometer used in this work.

REFERENCES

1.

2.

3.

4.

5. 6.

7.

8.

9.

10.

11.

E.g. (a) Nuclear Magnetic Resonance, Specialist Periodical Reports, Vols. 1-11. Chemical Society and Royal Society of Chemistry, London (1972-82); (b) J. Kowalevski, Prog. Nucl. Magn. Reson. Spectrosc. edited by J. W. Emsley, J. Feeney and L. H. Sutcliffe, Pergamon Press, Oxford, 11, 1 (1979); (c) M. Barfield and M. D. Johnson, Chem. Rev. 73, 53 (1973). R. C. Weast (Ed.), CRC Handbook of Chemistry and Physics, p. E64. Chemical Rubber Company Press, Boca Raton, FL (1980). V. S. Watts and J. H. Goldstein, J. Phys. Chem. 70, 3887 (1966). S. Watanabe and 1. Ando, Bull. Chem. SOC. Jpn. 53, 1257 (1 980). D. F. Evans, J. Chem. SOC. 5575 (1963). R. D. Green, Hydrogen Bonding by C-H Groups. Macmil- Ian, London (1974). J. B. Stothers, Carbon-13 NMR Spectroscopy. Academic Press, New York (1972). J.-R. Llinas, E.-J. Vincent and G. Peiffer, Bull. SOC. Chim. Fr. 11, 3209 (1973). and references cited therein. G. Govil and H. J. Bernstein, J. Chem. Phys. 47, 2818 ( 1967). H. S. Gutowsky, J. Jonas, F. Chen and R. Meinzer, J. Chem. Phys. 42, 2625 (1965). J. L. Marshall, Carbon-Carbon and Carbon-Proton NMR

Couplings. Verlag Chemie, Deerfield Park, FL (1983), and references cited therein.

12. J. W. Emsley, L. Phillips and V. Wray, Prog. Nucl. Magn. Reson. Spectrosc., edited by J. W. Emsley, J. Feeney and L. H. Sutcliffe, Pergarnon Press, Oxford, 10, 83 (1978).

13. W. T. Raynes and J. P. Riley, Mol. Phys. 27, 337 (1974). 14. C. Barbier, H. Faucher and G. Berthier, Jheor. Chim. Acta

21, 105 (1971). 15. G. Herzberg, Molecular Structure and Molecular Spectros-

copy: 11; Infrared and Raman Spectra of Polyatomic Molecules, p. 333. Academic Press, New York (1945).

16. J. K. Wilmshurst, J. Mol. Spectrosc. 1, 201 (1957). 17. K. Ito and H. J. Bernstein, Can. J. Chem. 34, 170 (1956). 18. A. Y. Meyer and N. L. Allinger, Tetrahedron 31,1971 (1975). 19. G. A. Crowder and H. K. Mao, J. Mol. Struct. 18, 33 (1973). 20. D. G. Lister, J. N. Macdonald and N. L. Owen, Internal

Rotation and Inversion, pp. 128 and 131. Academic Press, London (1978).

21. K. Kamienicka-Trela, Spectrochim. Acta, Part A 36, 239 (1980).

22. J. C. J. Barna, DPhil Thesis, Appendix 1, University of Oxford (1981).

Received 27 March 1984; accepted (revised) 25 July 1984

MAGNETIC RESONANCE IN CHEMISTRY, VOL. 23, NO. 3, 1985 193