The effect of density and temperature on the hydroxyl proton chemical shift in liquid ethanol

JOURNAL

OF MAGNETIC

RESONANCE

23,455460

(1976)

The Effect of Density and Temperature on the Hydroxyl Proton Chemical Shift in Liquid Ethanol J. W. LINOWSKI, Department

of Chemistry,

School University

NAN-I

of Chemical of Illinois,

LIU, AND J. JONAS

Sciences and Materials Urbana, Illinois 61801

Research

Laboratory,

Received September 18,1975 ; revision received March 20,1976 By employing the Fourier transform method, the internally referenced chemical shifts of the protons of absolute ethanol at temperatures between -83 and 120°C and pressures up to 5 kbar have been measured. Corresponding densities of absolute ethanol between 23 and 120°C are also reported for pressures up to 4 kbar. A substantial portion of the temperature shift of the hydroxyl proton is found to be due to density variation. An essentially linear change of the hydroxyl shift with density at five constant temperatures can be described by an empirical equation valid only for liquid ethanol relating the relative chemical shift of the hydroxyl proton to temperature and density. INTRODUCTION

has become a well-established method for probing the dynamic and static structure of liquids. A large number of NMR experiments have been performed employing temperature as a variable. Although it is well recognized that volume changes should have a substantial effect on liquid characteristics, concomitant changes in density with temperature variation are rarely investigated. If both pressure and temperature are employed as variables in an NMR experiment then it is possible in principle to separate density effects from kinetic effects due to temperature variation provided that density is also known as a function of pressure and temperature. The extent of the density effect on the NMR relaxation and self-diffusion of a number of systems (I) has recently been measured and found to be comparable in magnitude with the kinetic effect. However, for pressure range above 1 kbar few studies of the effect of density on the chemical shifts of dense liquids (24) have been reported to date. We are aware of no chemical shift studies in dense liquids in which systems with hydrogen bonding have been described as a function of density. The chemical shift of absolute ethanol was one of the first studied as a function of temperature (5). It was chosen for this preliminary investigation of the density influence on hydrogen bonded liquids for several reasons. The presence of methylene and methyl protons considerably simplified the experiment since the hydroxyl shift can be compared with both resonances, eliminating both external and internal references. Both bulk magnetic susceptibility of the medium and anisotropy in the molecular susceptibility of the solvent can be disregarded. Second, the relative shift of methyl to methylene groups shows little variation with temperature (5) and the absolute temperature shift of the methyl group (6) is anticipated to be an order of magnitude less Nuclear

magnetic

resonance

Copyright 0 1976 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain

455

456

LINOWSKI,

LIU

AND

JONAS

than that displayed by the hydroxyl protons. Experiments using external standard (7) indicated no detectable variation in the methyl-methylene chemical shift. One of the purposes of this work is to illustrate the performance of our NMR spectrometer system (8) to measure high-resolution NMR spectra of liquids under pressures above 1 kbar. EXPERIMENTAL

Absolute ethanol purchased from US1 Company was thoroughly dried over molecular sieves, then distilled in zxzcuofrom dry calcium sulphate. Oxygen was removed by multiple freeze-pump-thaw cycles. A Varian V-3800-1 electromagnet with a 3.8-inch air gap was used to accommodate the high-pressure vessel made of titanium alloy IMI 680 (Imperial Metal Industries, England). Pressure produced by Enerpac hand pumps in conjunction with a hydraulic intensifier was measured by a Heise-Bourdon gauge. A Lauda Kryomat was used for temperatures below room temperature, whereas high temperatures to 120°C were provided by a heating jacket with a resistance wire heater noninductively wound. In both cases temperature was controlled to better than f 1“C. A mixture of carbon disulfide and carbon tetrachloride four-to-three by volume was employed as a pressuretransmitting fluid. The degassed ethanol was transferred into a glass sample vessel which was connected to stainless steel bellows in an oxygen- and moisture-free glove box. The sample vessel was enclosed in a single coil rf insert made of DuPont Vespel high-temperature polyimide polymer. Data were collected by a Nicolet 1074 signal averager (Nicolet Instrument Corporation, Madison, Wis.) which was interfaced with a PDP-8E computer (Digital Equipment Corporation, Maynard, Mass.) where Fourier transforms were calculated with Nicolet-supplied software. Details of the high-pressure spectrometer have been published elsewhere (8). Spectral resolution of 2-8 Hz (linewidth at half-height) was maintained at all temperatures and pressures. Reproducibility of relative chemical shifts was 0.5 % or better. The density of ethanol was measured from 21 to 120°C and at pressures up to 4 kbar with a Bridgeman-type bellows piezometer (9) as modified by Cutler et al. (20). RESULTS

AND

DISCUSSION

The chemical shift of the ethanol hydroxyl proton referenced against the ethanol methyl group protons (4J is given in Table 1 at eight temperatures from -83 to 120°C and at pressures from 1 bar to 4 kbar. Values are reproducible to kO.5 % and are consistent with hydroxyl-methylene chemical shifts since the methyl resonance (6& does not discernibly shift with respect to the methylene resonance. Within the temperature and pressure limits of this experiment A,, remains constant at 2.29 ppm. At 1 atm and lower temperatures there is good agreement between these data and those published earlier by Feeney and Walker (II). At elevated temperatures there is progressively more disparity. As reported previously (5) increasing the temperature at 1 atm causes the ethanol hydroxyl proton to shift upfield toward the methyl resonance. This is found to be the case at all pressures studied. At any temperature, as the pressure is increased the hydroxyl proton shifts downfield away from the methyl resonance. The rate of this

DENSITY

EFFECTS

ON CHEMICAL TABLE

THE PRFSSURE

SHIFT

457

IN ETHANOL

1

Al3 (ppm)

AND TEMPERATURE DEPENDENCE OF THE CHEMICAL SHIFT DROXYL PROTON AND THE METHYL PROTON IN LIQUID

BETWEEN

THE HY-

ETHANOL

T (“Cl P (bar)

1

500 1000 1500 2000

a Extrapolated

-83

-52.5

-18

21.5

51.5

71

98.5

120

5.17 5.17 5.17 5.17 5.18 -

4.84 4.85 4.86 4.86 4.87 4.88 4.89 4.90 4.91 -

4.53 4.55 4.56 4.58 4.59 4.61 4.63 4.64 4.66 -

4.17 4.21 4.22 4.24 4.25 4.27 4.29 4.31 4.33 -

3.83 3.87 3.89 3.92 3.95 3.98 4.00 4.03 4.06 -

3.54” 3.60 3.65 3.68 3.71 3.75 3.77 3.79 3.82 -

3.18“ 3.25 3.31 3.36 3.42 3.46 3.49 3.52 3.55 3.58 3.61

2.82” 2.94 3.02 3.09 3.14 3.19 3.22 3.25 3.28 3.31 3.34

values.

shift is greatest at high temperatures and decreases with temperature until at -83°C there is no apparent change in Al3 with pressure. Of more theoretical interest is the variation in Al3 with density. Measured densities of absolute ethanol as a function of temperature (23 to 120°C) andipressure (1 bar to 4 kbar) are expressed in terms of Tait’s equation (22),

where p,, is the density (in g/cm3) at some pressure P (in bars), and pr is:the reference density at a reference pressure P, (in bars). The resulting Tait parameters B (bars) and C are found in Table 2. TABLE PARAMETERS

P Range (bars) 23.0 54.5 80.5 98.0 121.0 L?The parameters b Experimental authors (J.J.).

14000 l-4000 14000 l-4000

lb4000 reproduce density data

2

FOR THE TAIT

p, (bars) 1 1 1 1 1

EQUATION”,b

PC

B

(g/cm”)

(bars)

0.7884 0.7591

0.7394 0.7239 0.6977

the experimental may be obtained

744 657 573 568 391

densities to within on request from

C 0.1950 0.2047 0.2167 0.2277 0.2145 *0.5%. one of the

458

LINOWSKI,

LIU

AND

JONAS

When AI3 is plotted versus density at constant temperature, a series of straight lines results, as shown in Fig. 1. With increasing density at constant temperature the hydroxyl resonance shifts more downfield as the temperature is increased, indicating that density has more effect at high temperatures. An empirical equation relating the variation of d,, to temperatures between 21 and 120°C and at densities corresponding to

DENSITY

FIG.

p(g/

cm3)

1. The density dependence of AI3 (ppm) in liquid ethanol at several temperatures.

pressures of 1 bar to 4 kbar, valid for liquid ethanol in this restricted range of experimental conditions, is obtained from Fig. 1, A,, = (0.012t + 0.81)~ + 0.021t + 2.9,

PI

where AI3 is in parts per million, t is the temperature in degrees centigrade, and p is the density in grams per cubic centimeter. To separate the temperature and volume effects on the chemical shift, we present Table 3, which gives the values of (ErdJa~)~ and (ad,,/U), for several temperatures and densities. It is clear that during an isobaric experiment the chemical shift is affected TABLE PRESSURE AND TEMPERATURE

TV-2

22 52 71 98 120

both by between at 1 bar variation

3 DEPENDENCE

pWm3)

0.96 1.38 1.56 2.12 2.24

0.90 0.85 0.80 0.75 -

OF AIs

(aAG/aTIP (ppm/“C)

0.0102 0.0107 0.0116 0.0121 -

the change in temperature and the change in density. For example at 1 bar 23 and 120°C the change in Al, is found to be 1.34 ppm. The density of ethanol and 23°C is 0.79 g/cm3 and SO at this constant density the same temperature produces a change in Al3 of 1.14 ppm. Therefore, 0.2 ppm is due to the volume

DENSITY

EFFECTS

ON CHEMICAL

SHIFT

IN ETHANOL

459

change in this temperature interval. It is interesting to note a relatively small contribution of the density effect (-15%) in comparison to the thermal effect. In contrast, many dynamic properties of molecular liquids are very sensitive to density changes. The dominant effect of temperature may be connected with the fact that we are dealing with a hydrogen-bonded liquid. Even in their dynamic properties, water and heavy water show less sensitivity to volume changes when compared to the behavior of typical molecular liquids with no specific intermolecular interactions (I). Recent experiments (7) using an external reference at constant temperature and pressure verify the assumption that methyl and methylene groups exhibit essentially no chemical shift variation with density change. The magnetic screening constant of a molecule in a liquid medium has contributions from different sources (13-15), 0 (medium) = crB+ dA + gE + uW + cs,

131

where cB is the contribution proportional to bulk magnetic susceptibility of the medium; the second term, c,, arises from anisotropy in the molecular susceptibility of the solvent molecules. Since our experiments use relative chemical shifts between the hydroxyl and methyl group protons these first terms need not be considered further. The contribution o,, is due to polarization of the medium by the permanent dipole moment of the solute, and the term cW, arises from London dispersion forces. The last term, o,, which is important for our system studied, comes from specific intermolecular interactions such as hydrogen bonding. From Eqs. [2] and [3] one can write

A,, = a,(OH) - o&H,)

+ adOH)

- o,(CH,)

+ as(OH) - o,(CH,).

[41

It is quite clear from this expression that a quantitative analysis of the experimental data for the complicated hydrogen-bonded system of liquid ethanol is very difficult if not impossible. Therefore, only a qualitative, brief discussion of the trends in the experimental chemical shift of the hydroxyl proton with density and temperature is presented. Earlier results from this laboratory (4) indicate that changes in GEand oWwith density are in agreement with the experimental findings that the chemical shift of the hydroxyl proton in ethanol shifts to lower field with increasing pressure (density). However, on the basis of extensive experimental studies (6, II, 16,17) of the temperature dependence of chemical shifts in hydrogen-bonded systems, it may be possible that the contribution due to specific interactions us dominates over the other contributions. For ethanol it is well known (II) that both increased temperature and dilution by inert solvent produces an upfield shift of the hydroxyl proton, reflecting an increased shielding of that proton. The prevalent interpretations of these changes are based on changes in the average degree of association favoring a large fraction of disrupted or -distorted hydrogen bonds at higher temperatures and/or higher dilution with inert solvent. In our study we see that increasing density has just the opposite effect on the hydroxyl proton chemical shift than that observed for temperature changes. This would be compatible with the view that increasing density at all temperatures favors formation of a more extensive hydrogen bond network. However, on the basis of the experimental data obtained in this study we are unable to explain the origin of the density dependence of the chemical shift.

460

LINOWSKI,

LIU

AND

JONAS

Further experiments on other liquids over a wide range of experimental conditions are necessary before one attempts a detailed interpretation. ACKNOWLEDGMENTS Our thanks are due to the referee for his very valuable comments. We also wish to thank E. Alsmeyer and his co-workers at USI Company who analyzed our purified ethanol samples. This work was supported by the Energy Research and Development Agency under Contract E(ll-l)-1198. REFERENCES 1. J. JONAS, Ann. Rev. Phys. Chem. 26,167 (1975). 2. G. B. BENEDEK, R. ENGLMAN, J. A. ARMSTRONG, J. Chem. Sot. 96,1935 (1974). 3. H. YAMADA, T. ISHIHARA, AND T. KINUGASA, J. Amer. Chem. Sot. 96,1935 (1974). 4. D. WILBUR AND J. JONAS, J. Mugn. Resonance 10,279 (1973). 5. J. T. ARNOLD, M. E. PACKARD, J. Chem. Phys. 19,1608 (1951). 6. R. A. MEINZER, Ph.D. Thesis, University of Illinois, 1965. 7. J. W. LINOWSKI AND J. JONAS, unpublished results. 8. J. JONAS, Rev. Sci. Instrum. 43, 643 (1972). 9. P. W. BRIDGEMAN, Proc. Amer. Acad. Sci. 66,185 (1931). IO. W. G. CUTLER, R. H. MCMICKLE, W. WEBB, AND R. W. SCHIESSLER,J. Chem. Phys. 29,727 (1958). II. J. FEENEY AND S. M. WALKER, J. Chem. Sot. A 1148 (1966). 12. J. F. SKINNER, E. L. CUSSLER, AND R. M. Fuoss, J. PhyJ. Chem. 72,1057 (1968). 13. W. T. RAYNES, A. D. BUCKINGHAM, AND H. J. BERNSTEIN, J. Chem. Phys. 34,1084(1961). 14. F. H. A. RUMMENS AND H. J. BERNSTEIN, J. Chem. Phys. 43,297l (1965). IS. A. D. BUCKINGHAM, Canad. J. Chem. 38,300 (1960). 16. J. C. HINDMAN, J. Chem. Phys. 44,4582 (1966). 17. M. SAUNDERS AND J. B. HYNE, J. Chem. Phys. 29,13 19 (1958).