Formation and thermochemical properties of oxychlorides of niobium (Nb) and tantalum (Ta): Towards the gas-phase investigation of dubnium (Db) oxychloride

Article abstract of DOI:10.1016/j.ica.2018.10.032

The formation of NbOCl₃ and TaOCl₃ and their adsorption behavior on quartz surfaces was explored by applying an isothermal gas-chromatographic method. Trace amounts of short-lived Nb and Ta isotopes were used. Adsorption enthalpy values (ΔH_ads) at zero surface coverage of –ΔH_ads(NbOCl₃) = 102 ± 4 kJ/mol and –ΔH_ads(TaOCl₃) = 128 ± 5 kJ/mol were determined by analyzing the chromatographic behavior of the Nb and Ta complexes with a Monte-Carlo simulation method based on an adsorption–desorption kinetic model. By applying an empirical correlation, the experimental ΔH_ads values were successively related to the macroscopic standard sublimation enthalpy, ΔH^°_subl, as a measure of the volatility of each substance. The inferred sublimation enthalpies are in agreement with tabulated thermochemical values. Thus, the linear empirical correlation between ΔH_ads and ΔH^°_subl for metal-oxychlorides was updated with the inclusion of the present data. According to the predicted ΔH^°_subl(DbOCl₃), a ΔH_ads(DbOCl₃) value of 135 ± 2 kJ/mol was extrapolated. The future accomplishment of comparative studies with DbOCl₃ under the same experimental conditions will provide valuable information on the volatility trend in Group-5 elements, together with an indication on the magnitude of relativistic effects on the electronic structure of dubnium.

Full text of DOI:10.1016/j.ica.2018.10.032

InorganicaChimicaActa486(2019)361–366
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Formation and thermochemical properties of oxychlorides of niobium (Nb)
and tantalum (Ta): Towards the gas-phase investigation of dubnium (Db)
oxychloride
⁎
Nadine M. Chieraa, , Tetsuya K. Satoa, Tomohiro Tomitsukaa,b, Masato Asaia, Hayato Suzukia,c
Katsuyuki Tokoia,c, Atsushi Toyoshimaa, Kazuaki Tsukadaa, Yuichiro Nagamea,c
,
a Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
b Graduate School of Science and Technology, Niigata University, Niigata 910-2181, Japan
c Graduate School of Science and Engineering, Ibaraki University, Mito, Ibaraki 310-8512, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
The formation of NbOCl3 and TaOCl3 and their adsorption behavior on quartz surfaces was explored by applying
an isothermal gas-chromatographic method. Trace amounts of short-lived Nb and Ta isotopes were used.
Niobium
Tantalum
Adsorption enthalpy values (ΔHads
–ΔHads(TaOCl3) = 128
)
at zero surface coverage of –ΔHads(NbOCl3) = 102
4 kJ/mol and
Dubnium
5 kJ/mol were determined by analyzing the chromatographic behavior of the Nb and
Oxychlorides
Adsorption enthalpy
Sublimation enthalpy
Ta complexes with a Monte-Carlo simulation method based on an adsorption–desorption kinetic model. By
applying an empirical correlation, the experimental ΔHads values were successively related to the macroscopic
standard sublimation enthalpy, ΔH°subl, as a measure of the volatility of each substance. The inferred sublimation
enthalpies are in agreement with tabulated thermochemical values. Thus, the linear empirical correlation be-
tween ΔHads and ΔH°subl for metal-oxychlorides was updated with the inclusion of the present data. According to
the predicted ΔHs°ubl(DbOCl3), a ΔHads(DbOCl3) value of 135
2 kJ/mol was extrapolated. The future accom-
plishment of comparative studies with DbOCl3 under the same experimental conditions will provide valuable
information on the volatility trend in Group-5 elements, together with an indication on the magnitude of re-
lativistic effects on the electronic structure of dubnium.
1. Introduction
“one-atom-at-a-time” scale [9] begins with the selection of a proper
chemical system. The transactinide Dubnium (Db, Z = 105) belongs to
Recently, the completion of the 7th row of the Periodic Table was
achieved [1]. Since the 1960′s, several successful experiments demon-
strated that Superheavy Elements (SHEs) - i.e., elements with atomic
number Z > 103, also referred to as transactinides - behave chemically
similar to their lighter homologues in the corresponding group of the
Periodic Table. However, with increasing nuclear charges, the influence
of relativistic effects on the electronic structure causes deviations from
the periodicity of chemical properties [2–4]. A better understanding of
relativistic effects is essential for an accurate description and prediction
of the physical and chemical properties of heavy element atoms, clus-
ters, complexes, or even molecular structures [5–8]. The strong pro-
nunciation of relativistic effects on the chemical behavior of SHEs
renders these elements limiting benchmark cases.
Group-5 elements of the Periodic Table. It is well-known that its lighter
homologues niobium (Nb, Z = 41) and tantalum (Ta, Z = 73) exhibit a
strong tendency to react with halogens in the gas-phase, forming penta-
coordinated compounds. In the presence of oxygen, oxychlorides and
oxybromides species can be formed as well. Due to their volatility, the
resulting (oxy)halides are particularly suitable for gas-chromatographic
experiments. Up to now, comparative gas-phase studies with Nb, Ta,
and Db were focused on the formation and the chemical characteriza-
tion of their bromides and chlorides [10–14]. However, in the experi-
ments conducted to-date discordant results were obtained, summarized
in [15,16]. In those studies, the simultaneous formation of halides and
oxyhalides with oxygen traces in the carrier-gas, together with poor
chromatographic separations, did not allow for a proper chemical
characterization of the studied species [16–18].
A successful investigation of the chemical properties of SHEs at the
⁎ Corresponding author at: Research Group of Heavy Element Nuclear Science, Advanced Science Research Center, Japan Atomic Energy Agency, Ooaza Shirakata
2-4, Tokai, Naka, Ibaraki 319-1195, Japan.
E-mail address: nadinemariel.chiera@student.unife.it (N.M. Chiera).
https://doi.org/10.1016/j.ica.2018.10.032
Received 15 August 2018; Received in revised form 10 October 2018; Accepted 18 October 2018
Availableonline19October2018
0020-1693/©2018ElsevierB.V.Allrightsreserved.

N.M. Chiera et al.
InorganicaChimicaActa486(2019)361–366
Relativistic calculations predicted a higher stability for Nb-, Ta-, and
(
19F, xn)170Ta [20], respectively, at the JAEA Tandem accelerator. For
Db-oxychlorides in comparison to the corresponding chlorides and
oxybromides [18]. Hence, an Isothermal Gas-Chromatographic (IGC)
setup – devoted exclusively to the chemical exploration of oxychloride
species on quartz surfaces – was developed at the Tandem Accelerator
facility, Japan Atomic Energy Agency (JAEA). The retention behavior
of single molecules (i.e., trace-amount scale) of NbOCl3 and TaOCl3 on
quartz surfaces at different temperatures was explored, as a model
study for a future experiment with DbOCl3. As in matrix isolation ex-
periments, in “one-atom-at-a-time” gas-phase studies unstable mole-
cules can be easily analyzed since disproportionation reactions and
interactions between micro-components do not occur, allowing thus for
deducing thermochemical properties otherwise not accessible with
macro-amounts of substances [9,17,19]. In microscale chemistry, a
degree of volatility for a certain species is given by its adsorption en-
the purpose, mixed targets of natGe and natGd (thickness: 1 mg/cm2 and
0.4 mg/cm2, respectively) were used. The radionuclides produced in
the nuclear fusion reactions were thermalized and transported from the
target chamber to the quartz column (i.d.: 4 mm) by a He/N2 carrier
gas-jet (90:10 mixture, 1 L/min gas flow). The oxychlorination reaction
took place in Section (I) of the quartz column (length: 216 mm, i.d.:
4 mm), kept at a constant temperature by the reaction oven. In search
for the optimal oxychlorination conditions, the temperature of the re-
action oven was varied between 1073 and 1273 K. SOCl2 vapors in a
N2/O2(1%) gas stream (flow rate = 0.2 L/min) were used as oxychlor-
inating agents. Non-volatile products were retained on the quartz wool
plug placed in the middle of the oxychlorination section. The volatile
oxychloride species were transported to Section (II) of the quartz
column (length: 520 mm; i.d.: 4 mm) by the carrier gas, where the
isothermal chromatography was conducted. The temperature of Section
(II) was varied from 423 to 873 K. Temperature profiles inside Section II
(with Section I kept at 1273 K) for each of the isothermal settings from
473 to 873 K in 50 K steps are shown in Fig. 1. Prior to the experiment,
the internal surface of the entire quartz column, i.e., Section (I) and
Section (II), was maintained for 3 h under working conditions at 873 K
to achieve an efficient chlorination of the silanol groups at each iso-
thermal setting [21,22]. In Section (II), the molecules interacted with
the column surface in numerous adsorption/desorption steps, with re-
tention times indicative of their volatility. The species exiting the gas-
chromatographic column were then turbulently mixed in the recluster
chamber with an inert gas stream loaded with aerosol particles. The
aerosol particles were generated by sublimation of the correspondent
salt on a quartz boat placed inside a tubular furnace. The recluster
chamber was externally water-cooled to reduce losses of the in-
vestigated species by diffusion to its walls. To ensure statistical sig-
nificance, two experimental campaigns were conducted (namely, “Ex-
periments I” and “Experiments II”), which differed only from the
thalpy, ΔHads, on the chromatographic surface. This microscopic
quantity is temperature-independent and expresses the interaction en-
ergy between the adsorbate and the surface at zero surface coverage.
Here, we report the adsorption enthalpy values of NbOCl3 and TaOCl3
on the quartz chromatographic surface, deduced by analyzing their
chromatographic retention behavior. The experimental ΔHads values
were successively related to the macroscopic standard sublimation en-
thalpies, ΔH°subl. With the results obtained in this work, the empirical
correlation between ΔHads and ΔH°subl for metal-oxychlorides was up-
dated, allowing for the extrapolation of thermochemical values of
DbOCl3 with high accuracy.
2. Experimental
The exploration of NbOCl3 and TaOCl3 was performed with the IGC
setup schematized in Fig. 1. Short-lived isotopes of 88Nb (half-life t1/
2 = 14. 55 min) and 170Ta (t1/2 = 6.76 min) were simultaneously syn-
thesized in the nuclear fusion reactions natGe(19F, xn)88Nb and natGd
Fig. 1. Schematic of the Isothermal Gas-Chromatography (IGC) apparatus. See text for details.
362

N.M. Chiera et al.
InorganicaChimicaActa486(2019)361–366
TaCl3 + O2
TaOCl3 + O Hr = 10 kJ/mol Eact = 45.5 kJ/mol
(4)
ΔHr values were calculated with tabulated data from [25,26]. The
chlorination of NbOCl2 (and TaOCl2) and the oxidation of NbCl3 (and
TaCl3) are “fast enough” if the reaction time is lower than the half-life
of the radionuclide [23]. However, since t1/2 of 88Nb and 170Ta are in
the order of several minutes, the residence time of the reactive gases in
Section I of the column (tres) was considered here as “time limitation”:
Eact [J/mol]
10
< ln(n 10
tres)
Eq. (2)
RT
where n is the number of molecules of the reactant of interest (O2 or
Cl2) per unit of volume (in cm3) and tres = 0.135 s. Under our experi-
mental conditions (1 L/min He/N2 + 0.2 L/min SOCl2/N2(1%O2)) and
at T = 1073 K, only chlorinating reactions having an Eact < 220 kJ/
mol are “permitted”. Hence, already at T = 1073 K the chlorination of
NbOCl2 and TaOCl2 is kinetically promoted. For oxidation reactions at
T = 1073 K, the Eact upper limit is 180 kJ/mol, and thus, also the oxi-
dation of NbCl3 and TaCl3 is kinetically allowed. Since the bimolecular
reactions leading to the formation of NbOCl3 and TaOCl3 are kinetically
permitted, the lower yields observed at Treac < 1273 K suggest that at
those temperatures and under our experimental conditions, a less vo-
latile species is preferentially formed. The chlorinating agent Cl2 de-
rives from the pyrolysis of SOCl2 and its reaction with O2 at high
temperatures:
Fig. 2. Relative yields measured for volatile compounds of Nb and Ta as a
function of the reaction oven temperature. Dashed lines are provided to guide
the eye. During the measurements, the isothermal part of the quartz column
was kept at 873 K.
reclustering conditions applied (2.8 L/min KCl/He and 2.5 L/min CdI2/
N2 respectively). From the recluster chamber, the aerosol particles
carrying the Nb and Ta species were successively transported through a
25 m long capillary tube (i.d. = 2.4 mm) to the collection site in about
2 s. The aerosols were deposited on a glass fiber filter (8 × 12 mm,
0.5 mm thickness), and the γ-activities of 88Nb and 170Ta were mea-
sured with a HP-Ge detector. The glass fiber collector was changed
before each measurement.
3SOCl2 + 2O2
SO3 + 2SO2 + 3Cl2
(5)
According to speciation diagrams for the Nb–O–Cl and Ta–O–Cl
systems at T ≥ 773 K [27], pure Nb and Ta pentahalides cannot be
formed when the partial pressure of O2
(
PO2) is higher than 10–10 atm,
even under strong chlorinating conditions. This is consistent with [12],
where the synthesis of the solely NbCl5 could be observed only when
[O2] < < 1 ppm. According to the speciation diagrams at T = 773 K,
the direct chlorination of Nb and Ta species at logPO > -10 atm leads
2
3. Results and discussion
to the formation of NbO2Cl and TaO2Cl. However, at T = 1273 K, the
emergence of a NbOCl3(g) and TaOCl3(g) stability area is observed. It
follows that at this temperature, the formation of NbOCl3 and TaOCl3
over the synthesis of NbO2Cl and TaO2Cl can be promoted by slightly
varying the partial pressures of Cl2 and O2 in the system. In view of
these considerations, it can be assumed that at Treac = 1073 K under our
experimental conditions the formation of NbO2Cl and TaO2Cl is pre-
ferred, in agreement with the Ta–O–Cl and Nb–O–Cl speciation dia-
grams. By augmenting the temperature to 1173 K, an increase of the Cl2
vapor pressure is obtained, due to the promotion of reaction (5). As a
consequence, the synthesis of NbOCl3 is favored. Instead, at the very
same temperature the formation of TaOCl3 is still hindered by the
higher stability of TaO2Cl. This can be explained by the greater affinity
of tantalum towards oxygen. The preferential formation of TaOCl3 is
observed when the temperature is increased to Treac = 1273 K, in
agreement with the Ta–O–Cl diagrams. At this temperature an en-
hanced production of NbOCl3 is noticed as well. It has to be pointed out
that at Treac = 1273 K the simultaneous synthesis of both dioxychlor-
ides and oxytrichlorides species (even if in different amounts) cannot be
excluded. However, due to the observed higher volatility of NbOCl3 and
TaOCl3 in comparison to NbO2Cl and TaO2Cl [28,29], it can be assumed
that the solely species transported to our chemical setup are the oxy-
trichlorides.
3.1. Speciation of the studied Nb and Ta compounds
The yields of 88Nb and 170Ta deposited on the collection filters by
varying the temperature of the reaction oven (Treac) are depicted in
Fig. 2. The maximum yield was normalized to 100%. During the mea-
surements, the isothermal part of the chromatographic column was kept
at the maximum temperature (873 K). The highest yield of formation
and transport of volatile Nb and Ta species is observed at
Treac = 1273 K. In “one-atom-at-a-time” experiments, reactions between
two species at sub-tracer scale cannot be observed at the time scale of
laboratory experiments [9]. In this work, the concentrations of Ta and
Nb in each experiment are in the order of 10−21 M. Hence, dis-
proportionation, polymerization, or interactions between Nb/Nb, Ta/
Ta, Nb/Ta species are ruled out. Modification of the chromatographic
surface by the Nb- and Ta-oxychloride species is excluded as well. It is
reasonable to presume that the oxychlorination of Nb and Ta in the gas
phase proceeds through several bimolecular steps [23]. Since these
bimolecular reactions are characterized by a small entropy change, it is
possible to refer just to the standard enthalpy changes (ΔHr). A general
correlation between activation energy (Eact) and ΔHr for bimolecular
exothermic or endothermic reactions can be found in [24]:
Eact = 43 + 0.25 Hr [k J/mol]
Eq. (1)
3.2. Adsorption enthalpies of NbOCl3 and TaOCl3 on quartz
Hence, for the bimolecular reactions determining the formation of
NbOCl3 and TaOCl3 we obtain:
The yields of NbOCl3 and TaOCl3 passing through the gas-chroma-
tographic column as a function of the applied isothermal temperature
(Tiso) are shown in Fig. 3. The maximum yield was normalized to 100%.
Compared to NbOCl3, the TaOCl3 species becomes volatile at an ap-
proximately 100 K higher, indicating a lower volatility for the tantalum
oxychloride. In order to deduce the ΔHads of NbOCl3 and TaOCl3 on
NbOCl2 + Cl2
NbCl3 + O2
TaOCl2 + Cl2
NbOCl3 +Cl Hr=143 kJ/mol Eact = 79 kJ/mol
NbOCl3 + O Hr = 78 kJ/mol Eact = 63 kJ/mol
(1)
(2)
(3)
TaOCl3 + Cl
Hr = 126 kJ/mol Eact = 74.5 kJ/mol
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N.M. Chiera et al.
InorganicaChimicaActa486(2019)361–366
Table 1
Adsorption enthalpies of NbOCl3 and TaOCl3 on quartz surfaces determined in
this work and literature values. The arithmetic mean values are indicated in
italics.
Chemical species
−ΔHads (this work) kJ/mol
−ΔHads (literature) kJ/mol
(a)
(b)
NbOCl3
102
103
102
2
3
4
99
92
94
96
63
74
1
[12]
[32]
[33]
[34]
[35]
[36]
average
13
3
2
(a)
(b)
(c)
TaOCl3
126
130
128
1
5
5
157
133
60
12
20
5
[37]
[35]
[38]
average
149
(a)
(b)
(c)
Experiments I.
Experiments II.
Data from [39], reanalyzed in [17].
compilation of ΔHads(NbOCl3) and ΔHads(TaOCl3) values from litera-
ture, are reported in Table 1. As it can be seen, the values of
ΔHads(NbOCl3) and ΔHads(TaOCl3) found in this work are in agreement
with [12,33,34] and [37], respectively. However, the spread of the
literature data is probably due to different analysis procedures, or to
dissimilar experimental conditions (e.g., modifications of the chroma-
tographic surface with aerosol particles, incomplete conditioning of the
column, etc.) [23]. Therefore, from the existing literature data it is not
possible to draw conclusions about the relative volatility of NbOCl3 and
TaOCl3. It is worth mentioning that an incomplete chlorination of the
silanols on the quartz surface can heavily affect the retention time, and
hence the deduced ΔHads values. In fact, the modification of the surface
with chlorinating agent such as SOCl2 leads to the formation of ^Si–Cl
groups, which are capable of adsorption of the studied molecules by the
following scheme [23]:
Si Cl + MOyCln
Si Cl MOyCln
(6)
where M = central (metal) atom. In tracer-scale experiments, this in-
teraction is assumed to be similar to the adsorption of a MOyCln mo-
lecule on the surface made of macro-amounts of the same compound.
However, the non-modification of the quartz surface leads instead to
the reaction with silanol groups as follows [31]:
Fig. 3. Relative yields measured in Experiments
I (upper panel) and in
Si OH + MOyCln
( Si O)MOyCln + HCl
1
(7)
Experiments II (lower panel) for NbOCl3 (filled squares) and TaOCl3 (open
circles) as a function of the isothermal column temperature. The Monte-Carlo
simulations (solid lines) were performed using the ΔHads values which best fit
the measured data. Grey areas represent the error on the ΔHads values.
Such chemical interactions tend to increase the retention time in
comparison to the purely adsorption on the chloride groups, giving
hence higher |ΔHads| values which are not indicative of a MOyCln
–
MOyCln interaction [23].
In this work, the thermochemical information obtained from two
separate experimental campaigns is – within the error limits - con-
sistent, indicating the replicability of the results with the IGC device.
Furthermore, since the ΔHads values for NbOCl3 and TaOCl3 were ob-
tained under identical experimental conditions and with the same data
analysis procedure, a direct comparison of their adsorption enthalpy
values can give a trustworthy evidence of their relative volatility.
quartz, the breakthrough curves were analyzed with a Monte-Carlo si-
mulation method based on a kinetic model for gas-adsorption chro-
matography [30]. This model uses a microscopic description of the
chromatographic adsorption–desorption process at the atomic scale. To
determine the ΔHads of the molecule of interest on the stationary sur-
face, all the experimental conditions and the physical data of the gas
and solid phase are used as an input, leaving the adsorption enthalpy
the only varied parameter. The actual temperature profiles in the
column, as well the different half-lives of the investigated species, are
included as well. In the simulations, the adsorption residence time is
calculated by a Frenkel-type equation. For the purpose, the value
2·10−13 s for the reciprocal of the Lindemann frequency, as reported in
[12,17], was used. For each value of ΔHads entered, the simulation
program yields a chromatogram which has to be compared to the ex-
perimental one. By successively applying a least squares method, the
curve which better fits the measured data is chosen, allowing for
evaluating the ΔHads(NbOCl3) and ΔHads(TaOCl3) on the chromato-
graphic surface. The resulting adsorption enthalpies, together with a
3.3. Sublimation enthalpies of NbOCl3 and TaOCl3
For some selected compounds it is possible to correlate the micro-
scopic adsorption enthalpy to the macroscopic standard sublimation en-
thalpy [40]. For chlorides and oxychlorides adsorbing on quartz surfaces,
a semi-empirical correlation can be found in [41]. However, in that
correspondence ambiguous data points for NbOCl3 and TaOCl3 were in-
cluded. The adoption of that relation for the evaluation of our experi-
mental data would thus lead to biased extrapolations. Hence, an empirical
linear relation (Pearson’s coefficient R = 0.981) exclusively for oxy-
chlorides was determined by correlating available literature data (Fig. 4):
364

N.M. Chiera et al.
InorganicaChimicaActa486(2019)361–366
Fig. 4. Linear correlation between experimental −ΔHads values for oxychlor-
ides adsorbing on quartz surfaces [46–48] and the corresponding tabulated
standard sublimation enthalpies ΔH°subl [40,45,49,50]. For −ΔHads(MoOCl3)
and –ΔHads(WOCl4), data from [51] and [52] was re-analyzed with the Monte-
Carlo simulation method.
Fig. 5. Updated linear correlation between experimental ΔHads values and ta-
bulated standard sublimation enthalpies ΔH°subl for oxychlorides. For references,
see Fig. 4. The ΔHads(NbOCl3) and ΔHads(TaOCl3) values obtained in this work
were included and correlated with the correspondent tabulated ΔH°subl data from
[25].
H°
=
(1.513 ± 0.01)· Hads (28.73 ± 2.6) [kJ/mol]
Eq. (3)
subl
4. Conclusions
In this empirical correlation, all the considered ΔHads values were
derived by using the same type of analysis procedure adopted in this
work (i.e., a kinetic model developed by Zvara employing a Monte
Carlo simulation technique). By applying Eq. (4), sublimation en-
thalpies for NbOCl3 and TaOCl3 were directly calculated from the
averaged ΔHads values obtained in both Experiments I and Experiments
In preparation for the chemical exploration of DbOCl3, isothermal gas-
chromatographic experiments with its lighter homologues NbOCl3 and
TaOCl3 were conducted. From Monte-Carlo simulations the adsorption
enthalpy
values
–ΔHads(NbOCl3) = 102
4 kJ/mol
and
–ΔHads(TaOCl3) = 128
5 kJ/mol were obtained. Since the thermo-
II,
i.e.,
ΔH°subl(NbOCl3)
=
126
10 kJ/mol
and
chemical information for NbOCl3 and TaOCl3 was determined under
strictly identical chemical conditions, data inconsistencies resulting from
different experimental procedures were eliminated. By employing available
literature data, an empirical correlation between the ΔHads of oxychlorides
adsorbing on quartz surfaces and the related macroscopic ΔH°subl values was
ΔH°subl(TaOCl3) = 165
11 kJ/mol. To validate the so-found ΔHs°ubl
values, a comparison with available tabulated data has to be performed.
The standard sublimation enthalpy of a given compound can be ob-
tained by calculating the difference between its standard formation
enthalpy in the gaseous state, ΔH°f,g, and its standard formation enthalpy
in the solid state, ΔHf°,s. In this manner, ΔH°subl(NbOCl3) = 128.5 kJ/mol
was deduced from tabulated data [25,42], in agreement with our ex-
perimental results. For TaOCl3, discordant thermodynamic information
is available. Experimentally, from chemical vapor transport reactions it
was found that −ΔH°f,g(TaOCl3) = 784 kJ/mol [44]. In the same work,
the formation enthalpy of TaOCl3 in the solid phase was deduced by
assuming that ΔHs°ubl(TaOCl3) ≈ ΔHs°ubl(NbOCl3) = 110 kJ/mol. This
imprecise value of –ΔH°f,s(TaOCl3) = 892 kJ/mol is still in use in
common databases such as [26,43], and [45]. Instead, a more accurate
–ΔH°f,s(TaOCl3) = 952 kJ/mol can be found in [25]. The latter was
obtained from the chemical stability at 298 K:
derived. This allowed for calculating ΔH°subl(NbOCl3) = 126
10 kJ/mol
and ΔH°subl(TaOCl3) = 165
11 kJ/mol, in good agreement with litera-
ture data. The validation of the experimental ΔHads values for NbOCl3 and
TaOCl3 permitted to update the empirical linear relation ΔHads vs. ΔH°subl
for oxychlorides. In [53], a sublimation enthalpy value for DbOCl3 of
180 kJ/mol was predicted. By applying the present linear correlation, a
value of ΔHads(DbOCl3) = 135
2 kJ/mol is extrapolated. Hence, the
expected sequence for the interaction strength with a quartz surface is
NbOCl3 < TaOCl3 < DbOCl3, with the transactinide complex being thus
the least volatile. The future chemical exploration of DbOCl3 with the IGC
setup will provide a final assessment on the relative volatility of Group-5
oxychlorides. Furthermore, the experimental ΔHads(DbOCl3) will allow for
an estimation of ΔH°subl(Db). Hence, the experimental results will provide
theoretical chemists a valuable information in order to understand the
magnitude of relativistic effects on the electronic structure of dubnium.
Ta2O5(s) + 3TaCl5(s)
5TaOCl3(s)
G° = 20.34 kJ/mol
(8)
It follows that ΔH°subl(TaOCl3) = 170 kJ/mol, in accordance with
our experimental result.
Acknowledgments
3.4. Updated empirical ΔHads vs. ΔH°subl correlation for oxychlorides
The authors express their gratitude to the crew of the JAEA tandem
accelerator for their assistance in the course of these experiments. The
present work was partly supported by JSPS Kakenhi Grant-in-Aid for
Young Scientists (B) [grant number 20740152].
In Section 3.3 the validity of the ΔHads for NbOCl3 and TaOCl3 ob-
tained in this work was demonstrated. Hence, these microscopic
quantities together with their corresponding tabulated ΔH°subl values can
now be used for an update of Eq. (3). The revised correlation is shown
in Fig. 5. The derived expression for the linear regression (Pearson’s
coefficient R = 0.987) is:
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2018.10.032.
H°
=
(1.590 ± 0.01)· Hads (34.68 ± 2.06) [kJ/mol]
Eq. (4)
subl
365

N.M. Chiera et al.
InorganicaChimicaActa486(2019)361–366
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