Enthalpies of Sublimation of Organic and Organometallic Compounds. 1910–2001 James S. Chickosa… Department of Chemistry, University of Missouri-St. Louis, Saint Louis, Missouri 63121 William E. Acree, Jr.b… Department of Chemistry, University of North Texas, Denton, Texas 76203 ͑Received 22 October 2001; accepted 11 February 2002; published 7 June 2002͒ A compendium of sublimation enthalpies, published within the period 1910–2001 ͑over 1200 references͒, is reported. A brief review of the temperature adjustments for the sublimation enthalpies from the temperature of measurement to the standard reference temperature, 298.15 K, is included, as are recently suggested values for several reference materials. Sublimation enthalpies are included for organic, organometallic, and a few inorganic compounds. © 2002 American Institute of Physics. Key words: compendium; enthalpies of condensation; evaporation; organic compounds; organometallic compounds; sublimation; sublimation enthalpy. Contents enthalpies to 298.15 K....................... 538 2. A hypothetical molecule illustrating the different hydrocarbon groups in estimating Cp........... 541 1. Introduction................................ 537 2. Heat Capacity Adjustments. ................. 538 3. Group Additivity Values for C (298.15 K) 1. Introduction pc Estimations................................ 539 Sublimation enthalpies are important thermodynamic 4. Reference Materials for Sublimation Enthalpy properties of the condensed phase. Frequently they are used Measurements.............................. 539 in correcting enthalpies of formation to the gas phase and in 5. Sublimation Enthalpy Compendium............ 539 evaluating environmental transport properties.1,2 Sublimation 6. References for Tables 6 and 7................. 667 enthalpy measurements are also useful to studies of polymor- 7. Acknowledgments.......................... 697 phism and predictions of molecular packing. The measure- 8. References................................. 697 ments provide benchmark numbers that can be used to vali- date the calculations.3 Examination of the data in this List of Tables compendium will reveal some large discrepancies in reported 1. Equations for the temperature adjustments of enthalpies of sublimation. It is likely that some of the dis- sublimation enthalpies....................... 538 crepancies reported by different laboratories are due to mea- 2. Group values for estimating the C (298.15 K)... 4 pc 540 surements made on different polymorphic modifications. 3. Some estimations of C (298.15 K) using the pc Sublimation enthalpy measurements also can reveal differ- group values of Tables 2͑A͒ and 2͑B͒.......... 542 ences in interactions in chiral solids and their racemic modi- 4. Recommended reference standards for fications. Very little experimental work has been reported in sublimation enthalpy measurements............ 543 this respect.5,6 5. A list of acronyms used in Tables 6 and 7....... 544 Our interests in sublimation enthalpies goes back nearly 3 6. Reported enthalpies of sublimation of organic decades.5 Initially interested in using sublimation enthalpies compounds, 1910–2001...................... 545 to correct enthalpies of formation data to a standard state, we 7. Enthalpies of sublimation of some organometallic have since focused our attention on their measurement,7 and inorganic compounds, 1910–2001.......... 633 estimation,8 and assessment.9 In a parallel study, a compila- tion of available sublimation enthalpies was initiated in the 5 List of Figures 1980s. A reasonably exhaustive version of this database 1. A thermodynamic cycle for adjusting sublimation covering the literature up to the mid 1990s is available on line at http://webbook.nist.gov/chemistry/. The present ver- sion updates this compilation to the year 2001. Although our a͒ Author to whom correspondence should be addressed; electronic mail: intent has been to provide an exhaustive coverage of the [email protected] b͒Electronic mail: [email protected] literature from 1910 to 2001, this listing is probably still far © 2002 American Institute of Physics. from complete. Õ Õ Õ Õ Õ 0047-2689 2002 31„2… 537 162 $35.00537 J. Phys. Chem. Ref. Data, Vol. 31, No. 2, 2002 538 J. S. CHICKOS AND W. E. ACREE, JR. ⌬ ͒ subHm͑298.15 K T ϭ⌬ ͑ ͒ϩ ͵ m ͑ Ϫ ͒ ͑ ͒ subHm Tm C p Cpg dT, 1 298.15 c ⌬ ͒ subHm͑298.15 K Ϸ⌬ H ͑T ͒ϩ͑C ϪCp ͓͒T Ϫ298.15͔. ͑2͒ sub m m pc g m Experimental heat capacities for many solids at 298.15 K are available.10 Experimental gas phase heat capacities for compound that are solids at 298.15 K are unavailable and generally need to be estimated. Gas phase heat capacities can FIG. 1. A thermodynamic cycle for adjusting sublimation enthalpies to 298.15 K. be calculated from statistical mechanics or estimated by group additivity methods.11 A number of group additivity methods have been developed to estimate gas phase heat 2. Heat Capacity Adjustments capacities.11–13 However, group values for some functional groups are not available. This has encouraged the develop- Sublimation enthalpies are measurements based on mass ment of other estimation methods. Table 1 briefly summa- transport and as such are directly or indirectly dependent rizes the various equations that have been used in place of upon vapor pressure. The vapor pressure of different solids at the second term in Eq. ͑2͒. the same temperature can vary by many orders of magnitude. Equation ͑3͒ can easily be derived by assuming that the In order to obtain a reasonable amount of mass transport, it is gas is ideal and that the Dulong–Petit value of 3RN holds for frequently necessary to conduct these measurements at tem- the solid, where the term R represents the gas constant and N 5 peratures that differ substantially from the standard reference is the number of atoms/molecule. A similar relationship but temperature, 298.15 K. The actual temperature of measure- characterized by a temperature coefficient of 6R ͓Eq. ͑4͔͒ has 14 ment depends on the sensitivity of the instrument or appara- been suggested by Pedley. Temperature coefficients of 40 Ϫ1 15 tus and the properties of the substance. In addition, these J mol have been used by Melia and Merrifield, and a Ϫ1 16 measurements are often conducted as a function of tempera- value of 60 J mol has been used by de Kruif et al. for a ture. series of amino acids and peptides. The magnitude of the sublimation enthalpy is dependent A major limitation of most of the equations listed in Table on temperature. Figure 1 and Eqs. ͑1͒ and ͑2͒ illustrate the 1 is that the heat capacity adjustments are treated as universal origin of this temperature dependence in terms of a thermo- constants independent of molecular structure. Only Eq. ͑7͒ is dynamic cycle. If the heat capacities of the solid and gas sensitive to differences in molecular structure. This equation phase are known, C p and Cpg , respectively, then the subli- was derived from a correlation using estimated heat capaci- c 17 mation enthalpy at 298.15 K can be related to the experimen- ties of the solid at 298.15 K. This correlation was devel- ͑ ͒ oped from the observed dependence of the temperature ad- tal measurements by using Eq. 1 . This equation, generally 17 referred to as Kirchhoff’s equation, can be used to adjust justment on both molecular structure and size. Experimental or estimated values of C (298.15 K) can be sublimation enthalpy measurements to any reference tem- pc perature. Tm represents either the temperature of measure- used in this equation. ment for calorimetric measurements or the mean temperature Previous work has demonstrated that Eq. ͑7͒ gives results of measurement for experiments conducted over narrow that are generally as good as or better than the use of the 7,8͑b͒,18 ranges of temperature. Treating the heat capacities of the two other equations in Table 1. The use of Eq. ͑7͒ should phases as independent of temperature and integrating Eq. ͑1͒ be limited to the temperature range 200–500 K. A standard Ϫ1 results in Eq. ͑2͒. Since the magnitude of the heat capacity of deviation of Ϯ33 J mol has been associated with the term: ͓0.75ϩ0.15C (298.15 K)͔. The total uncertainty of the the gas phase is usually smaller than that of the solid phase pc ͑c͒, sublimation enthalpies increase with decreasing tempera- temperature adjustment depends on both the magnitude of ture C and T . In applications, an uncertainty of one-third of pc m TABLE 1. Equations for the temperature adjustments of sublimation enthalpies Corrections for the sublimation enthalpies ͑JmolϪ1͒ Equation Reference (C ϪCp )͓T Ϫ298.15͔ϭ2R͓T Ϫ298.15͔ ͑3͒ 5 pc g m m (C ϪCp )͓T Ϫ298.15͔ϭ6R͓T Ϫ298.15͔ ͑4͒ 14 pc g m m (C ϪCp )͓T Ϫ298.15͔ϭ40͓T Ϫ298.15͔ ͑5͒ 15 pc g m m (C ϪCp )͓T Ϫ298.15͔ϭ60͓T Ϫ298.15͔ ͑6͒ 16 pc g m m (C ϪCp )͓T Ϫ298.15͔ϭ͓0.75ϩ0.15C (298.15 K)͔͓T Ϫ2.98͔ ͑7͒ 17 pc g m pc m J. Phys. Chem. Ref. Data, Vol. 31, No. 2, 2002 ENTHALPY OF SUBLIMATION 539 the total temperature adjustment has been arbitrarily chosen pounds exhibiting low vapor pressures. While some of the as the uncertainty ͑Ϯ2 ␴͒.9 observed differences in reported enthalpies may be due to Equations ͑3͒–͑6͒ do not require C values; their use can polymorphism, others are probably due to the lack of a suf- pc be an advantage if an appropriate group value or experimen- ficient number of reference compounds that vary in their tal heat capacity is unavailable for a particular substance. range of volatility. The ability of an experimental technique Temperature adjustments to 298.15 K are often small and to measure vapor pressure in one pressure or temperature frequently of the same order of magnitude as the uncertainty region does not in itself guarantee the same accuracy in an- associated with the measurement. This is the rationale some other. Substantial variations in sublimation values are re- authors give for not adjusting the measurements for tempera- vealed in the tables that follow.