Deuterium isotope differences in 2-propanone, (CH3)2CO/(CD3)2CO: a high-pressure sound-speed, density, and heat capacities study

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Abstract

A new high-pressure, non-intrusive ultrasonic microcell [J. Chem. Thermodyn. 36 (2004) 211–222] was used to carry out sound-speed measurements in deuteriated 2-propanone (acetone-d6) in broad ranges of temperature (288 < T/K < 338) and pressure (0.1 < p/MPa < 160). To the best of our knowledge, there are no data regarding speed of sound of acetone-d6. (p, ρ, T) data for acetone-d6 were also determined but in a narrower T, p range (298 to 333 K; 0.1 to 60 MPa). In this interval, several thermodynamic properties were thus determined, such as: isentropic (κs) and isothermal (κT) compressibility, isobaric thermal expansivity (αp), isobaric (cp) and isochoric (cv) specific heat capacity, and the thermal pressure coefficient (γv). Comparisons with our data for acetone-h6 enabled us to establish the magnitude and sign of deuterium isotope effects for identical properties. These effects are a consequence of distinct vibrational mode frequencies in an isotope-invariant force constants’ field. Molar heat capacities and their isotope effects were theoretically determined by employing an Einstein-like model for the vibrational frequencies of acetone-h6 and acetone-d6.

Introduction

Very recent developments in the design, assembly, and tests of a novel sound-speed, non-intrusive microcell [1] have resulted in new measurements of the speed of propagation of ultrasound waves in perprotonated 2-propanone (acetone-h6), (CH3)2CO, over a broad temperature range and pressures up to 160 MPa. These data combined with new density measurements up to 60 MPa have led to accurate determinations of the isobaric and isochoric specific heat capacities and of some other thermodynamic quantities such as isothermal and isentropic compressibilities and the isobaric expansivity [1]. The comparison of our results with reliable literature data shows good agreement for all the above mentioned quantities. This fact, along with the high internal precision of our data, have prompted us to extend these measurements in order to detect a subtle effect, that of isotopic substitution. Hence, perdeuteriated 2-propanone (acetone-d6), (CD3)2CO, has been studied in this work.

Acetone is one of the most popular, important solvents but surprisingly one encounters a lack of comprehensive studies on its thermophysical properties. This is even more so for its deuteriated counterpart. Once this situation is addressed, it will be possible to make improvements in the practical use of both solvents, while also increasing our understanding of and ability to predict their behaviour via the use of molecular-based models. In this work, we report extensive data of several thermophysical properties of acetone-d6 in broad temperature and pressure ranges, which when combined with those of acetone-h6, permit us to test the reliability of a simple statistical-mechanics approach for heat capacitiy isotope effects. Unfortunately, in contrast to the situation encountered regarding many other properties, there is not, to date, a well-established theory for molar volume isotope effects [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. The rather murky procedure of establishing links between molecular structure and density combined with the lack of extensive (and accurate) data on the molar density shifts upon isotopic substitution are the responsible factors. We hope that this work may encourage further progress towards this goal.

To the best of our knowledge, there are no data of speed of sound of acetone-d6. Few laboratories have studied the influence of isotopic substitution on the thermodynamic properties of acetone. Van Hook and his collaborators [2] studied the vapour pressure isotope effect and the isotope effect (IE) on the molar volume as well as on the isothermal compressibility and isobaric expansivity [3]. The latter properties have been investigated over considerable temperature and pressure ranges. Brown [4] reported on the molar volume IE of acetone at 298 K and 0.1 MPa, while Ibberson et al. [5] determined calorimetrically the isobaric specific heat capacity IE in the temperature range (10 to 300) K and 0.1 MPa.

IEs on the macroscopic properties of condensed phases express quantum-mechanical isotopic differences (distinct vibrational states) in conveniently chosen intra- and inter-molecular isotope independent potential energy surfaces [6]. In other words, IEs are a consequence of distinct vibrational mode frequencies and vibrational frequency distributions in an isotope-invariant force constants’ field. While, generally, one can state that, condensed phase IEs are very well interpreted in the light of the quantum-mechanical statistical theory of IEs as originally formulated by Bigeleisen [7] and further developed by Stern et al. [8], volumetric and volume-related properties IEs are currently the least well understood of all IEs on equilibrium physical properties. Matsuo and Van Hook [9] thoroughly investigated the influence of isotopic substitution on the experimental volumetric properties of benzene, cyclohexane and their mixtures, interpreting it in a semi-quantitative fashion. In the same vein, Kooner and Van Hook [3] discussed the high temperature results obtained for acetone. In their case, the isotope effect on the molar voume at temperatures far from the critical one is primarily a consequence of the vibrational amplitude differences between the C–H and C–D stretching modes. Theoretical studies of low-temperature molar volume IEs of this same kind (mechanical in origin) have also been offered by Bartell and Roskos [10]. Alternatively, Bigeleisen and collaborators [11] used a statistical-mechanics approach. The isotope effect on the isothermal compressibility [9] has been analysed in terms of a model which partitions the effects into hard-sphere, librational, internal, and intermolecular attractive parts. This analysis showed that only hard-sphere and librational contributions are important. The isotope effect on the thermal expansivity appeared to be associated principally with the high frequency isotope sensitive vibrations (such as CH/CD stretches) and their anharmonicity.

This paper presents measurements of the speed of sound for deuteriated acetone in the temperature range (288 to 338) K at pressures up to 160 MPa and densities in the temperature range (298 to 333) K up to 60 MPa, and gives new results for the IE on several volumetric properties of acetone which are complementary to the high temperature data of Kooner and Van Hook [3] while extending the data to much higher pressure. These raw data have permitted us to accurately determine molar heat capacities, which are compared with the theoretical results obtained by employing an Einstein-like model for the vibrational frequencies.

Section snippets

Experimental

Deuteriated acetone (acetone-d6), 99.6 atom% D was obtained from Aldrich and used without any further purification. Measurements of the speed of sound (operational frequency of 0.5 MHz) in the temperature range (288–338) K and pressure range (0.1 to 160) MPa were carried out by using the same experimental setup as previously described in great detail [1] for the measurements of the speed of sound of protonated acetone. Accordingly, the same experimental procedure and calculations were also

Results and discussion

The speed of sound was measured between (288 and 338) K spanning the pressure range up to 160 MPa in a total of 106 data points. The experimental results of the speed of sound for acetone-d6 are collected in table 1 and presented in figure 1 for six isotherms as a function of pressure. The experimental data were fitted to a Pade 3 × 3 equation of the form(u/m·s-1)=i=02j=02aij(T/K)i(p/MPa)jk=02l=02bkl(T/K)k(p/MPa)l.The values of the coefficients were calculated by means of a least-squares

Conclusions

Our values of observed and calculated properties of acetone-d6 are generally in good agreement with the very limited data found in the literature for this isotopomer. Our data extends past the span of this type of study to a very broad range of applied pressures. Deuterium isotope effects in the condensed phase behaviour of acetone can be understood in the light of vibrational mode frequencies as well as amplitudes’ shifts upon isotopic substitution. Although isotope effects on several

Acknowledgements

This work was financially supported by Fundação para a Ciência e Tecnologia, Portugal, under contract POCTI/EQU/34955. R.G.A. and J.M.S.S.E. are grateful to Fundação para a Ciência e Tecnologia for doctoral fellowships.

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