Standard enthalpies of formation
445
180
170
160
150
140
130
interatomic distances exist, where their values are consid-
erable different, despite the fact that a correlation could be
established between both distances [27]. Their main
importance is the capacity to reproduce the lattice energies
through Eq. 4.
It is concluded that Eq. 4 reproduces the experimental
DlatU2o98ðMSRÞ data with an average absolute deviation of
9.8 kJ mol-1 and a maximum relative deviation of 3.9%.
The obtained DlatU2o98ðMSRÞ values were subsequently
used to estimate the enthalpies of formation of various
unmeasured metal thiolate compounds (RbSC2H5, MS-n-
C4H9; M = Li, K, Rb, Cs) by using Eq. 3. As mentioned
above, these results are also listed in Table 2. It should be
stressed that despite the fact the error associated with these
estimated values to be considerably larger than the one in
the case of the experimentally determined enthalpies, the
enthalpy of formation value should be a reasonable
approximation for the cases were experimental values are
not available. Moreover, this study shows that a simple
model, like the Kapustinskii one here described, can
accommodate a large variety of compounds, such as
alkoxides, phenoxides, thiolates, or MCp (Cp = cyclo-
pentadienyl) for alkaline and alkaline-earth metals. The
extension of such a model to higher valence state metals is
under study.
120
XH
X-n -C4H9
XC2H5
Fig. 2 Thermochemical ionic radii for thiolates (this work) and
alkoxides [27]. Radii for SR- (filled diamonds) and for OR- (open
diamonds)
DlatU2o98ðMSRÞ ¼ DlatU0oðMSRÞ þ ðH2o98 ꢁ H0oÞMþ
þ ðH2o98 ꢁ H0oÞSRꢁ ꢁ ðH2o98 ꢁ H0oÞ
ꢁ 2RT
MSRð5Þ
where R is the gas constant and T = 298.15 K. The infor-
mation needed to compute the correction term X ¼ ðH2o98
ꢁ
H0oÞMþ þ ðH2o98 ꢁ H0oÞSRꢁ ꢁ ðH2o98 ꢁ H0oÞMSR ꢁ 2RT is not
available for the thiolates studied in this work. A fairly
small value of X (comparable to the uncertainty that affects
most experimental values of the lattice enthalpy in Table 3)
is, however, expected. No values are available for MSH
also, but as an example, in the case of the Li, Na, and K
hydroxides, X = 2.43, -0.64, and -2.30 kJ mol-1
,
References
respectively [34]. Hence, in the following discussion it will
be assumed that, to a good approximation, DlatU2o98
ðMSRÞ ¼ DlatU0oðMSRÞ
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123