Formation of positive and negative ions in CH3NO2

Article abstract of DOI:10.1021/jp022031h

Absolute dissociative ionization cross-sections from threshold to 200 eV have been measured using Fourier transform mass spectrometry (FTMS). In the production of positive ions by electron impact ionization, 13 ions are detected, including the parent ion

Full text of DOI:10.1021/jp022031h

9040
J. Phys. Chem. A 2003, 107, 9040-9044
Formation of Positive and Negative Ions in CH3NO2
†
‡
,‡
C. Q. Jiao, C. A. DeJoseph Jr., and A. Garscadden*
InnoVatiVe Scientific Solutions Inc., Dayton, Ohio 45440-3638, and Air Force Research Laboratory,
Wright-Patterson Air Force Base, Ohio 45433-7251
ReceiVed: September 9, 2002; In Final Form: July 14, 2003
Absolute dissociative ionization cross-sections from threshold to 200 eV have been measured using Fourier
transform mass spectrometry (FTMS). In the production of positive ions by electron impact ionization, 13
+
+
ions are detected, including the parent ion CH
NO , NO , and CH
2 3
3
NO
. Kinetic energies of some selected positive ions are determined using the technique
of trapping potential dependency study in FTMS experiments. In the production of negative ions by dissociative
2
3
and the four most important fragment ions CH NO ,
+
+
+
-
-
-
2 2 2
electron attachment to nitromethane, three anions were detected: CH NO , NO , and CN . The high
sensitivity of the FTMS permits the subsequent time-resolved reactions of these anions to be studied at very
low pressure.
5
I. Introduction
cell (5 cm on a side) and a 2 T superconducting magnet. The
theory and methodology of FTMS have been well-documented
Nitromethane (CH3NO2) has been of great research interest
because of its use as a fuel and as a prototype molecule for a
class of high-energy materials. At high altitudes, uninhabited
air vehicles (UAVs) and other aircraft have had problems with
flameout and reignition. Solutions have included silane that is
pyrophoric, but it also produces abrasive silicon dioxide. Recent
approaches have used plasma igniters and fuels that contain
oxygen such as nitromethane. Plasma-assisted ignition is also
6
-8
in the literature.
CH3NO2 was purchased (96+%, Aldrich),
and in addition we applied multiple liquid N2 freeze-pump-
thaw cycles to remove noncondensable gases. For the specific
lot of CH3NO2 used in our experiments (Lot No. DO 0086PI),
the purity is 99.1% as determined by Aldrich using gas
chromatography, with the major impurity noted as H2O. Our
FTMS data indicate no impurities other than H2O at an estimated
1
3 2
0.2%. CH NO is mixed with argon (99.999%, Matheson) with
an approach being explored for scramjet flame holding. To
a ratio about 1:1 to a total pressure of ∼10 Torr, as determined
by capacitance manometry. The mixture is then admitted through
a precision leak valve into the FTMS system. Ions are formed
design such systems, one needs a database for the ionization
cross-sections of the fuel compounds. As a fuel, nitromethane
and other NO2 containing hydrocarbons have much shorter
ignition delays at a given temperature compared to the corre-
sponding hydrocarbon without an NO2 group. This result is
believed to be due to the production of alkyl radicals that
facilitate the ignition processes.2 In a parallel study, the
formation of negative ions in nitromethane has been measured.
It has been well-known for decades that nitromethane can be
sensitized significantly toward detonation by amines, but the
sensitization mechanism is not well-understood. A comprehen-
sive review of the mechanism of the amine sensitization of
shocked nitromethane can be found in recent studies by
-
7
by electron impact in the trapping cell at pressures in the 10
Torr range. An electron gun (Kimball Physics ELG2, Wilton,
NH) irradiates the cell with a few hundred picocoulombs of
low-energy electrons. The motion of the ions is constrained
radially by the superconducting magnetic field and axially by
an electrostatic potential (trapping potential) applied to the trap
faces that are perpendicular to the magnetic field. The trapping
potential is usually set to 10 V unless stated otherwise (see
below). Ions of all mass-to-charge ratios are simultaneously and
coherently excited into cyclotron orbits using stored waveform
3
4
9-11
Gruzdkov et al. and by Woods et al. In brief, a central issue
of the mechanism is whether and how an anion from ni-
inverse Fourier transform (SWIFT)
applied to two opposing
trap faces which are parallel to the magnetic field. Following
the cyclotron excitation, the image currents induced on the two
remaining faces of the trap are amplified, digitized, and Fourier
analyzed to yield a mass spectrum.
-
tromethane, CH2NO2 , is involved in the early stages of the
decomposition of the detonating nitromethane.
In this paper, we report our study on the positive and negative
ion chemistries in nitromethane using Fourier transform mass
spectrometry (FTMS). The cross-sections for electron impact
ionization of nitromethane and some measurements of the
energies of the ions fragments are presented. The ion-molecule
FTMS is an established technique for studying the kinetics
of charged particle reactions, in which the signal peak heights
12
are used to evaluate the number of ions in the cell. In this
+
study, the intensity ratios of the ions from CH3NO2 to Ar give
-
reactions involving CH2NO2 are discussed.
13
cross-sections relative to those for argon ionization since the
pressure ratio of CH3NO2 to Ar is known.
II. Experimental Section
A modification has been made recently to the trapping cell
of the Extrel FTMS, which consists of adding a pair of screen
electrodes in front of the trapping plates. A significant improve-
ment to the quality of the cross-section data has been achieved
by this modification: holding the screens to ground potential
produces the particle-in-a-box potential (rather than the harmonic
All of the experiments are performed using a modified Extrel
FTMS equipped with a cubic ion cyclotron resonance trapping
*
Corresponding author. Fax: 937-656-4657. E-mail: alan.garscadden@
wpafb.af.mil.
†
‡
Innovative Scientific Solutions.
Wright-Patterson AFB.
1
0.1021/jp022031h CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/02/2003

Positive and Negative Ion Formation in CH3NO2
J. Phys. Chem. A, Vol. 107, No. 42, 2003 9041
oscillator potential) along the z-axis of the trapping cell.14 Thus,
we can apply high trapping potentials to entrap more kinetically
energetic ions, while in most of the volume of the cell the
potential drop is small enough to avoid the broadening of the
electron energy distribution. Wang and Marshall15 have a cell
geometry similar to the one in our FTMS instrument and give
a detailed description of the design of the screen electrodes.
Each screen electrode was constructed from 0.0010 in. diameter
tungsten 50 × 50 mesh (Unique Wire Weaving Co., Inc.,
Hillside, NJ); two out of every three wires were then removed
to give a final mesh of 16/in. The mesh was held in place by
spot-welds to a 314 stainless steel frame that was positioned
1/8 in. inside of the adjoining trapping plate. When the trapping
potential is set to 10 V, as mentioned above, the potential drop
within the screen electrode region along the z-axis is estimated
to be 0.3 V.
For studies of the kinetic energy of ions generated by electron
impact ionization, the trapping potential dependence (harmonic
oscillator potential) of the ion intensity is determined, as
16
described previously. In these experiments, the screen elec-
trodes are connected to their adjoining trapping plates and the
trapping potential is varied from 0.40 to 10 V. Because of the
shape of the harmonic oscillator potential, this corresponds to
an actual well depth of the trap, Veff, varying from 0.26 to 6.6
V. If the ions are kinetically energetic, increasing Veff results in
a greater number of ions trapped in the cell and the plots of the
fraction of the ions trapped, f, versus the square root of Veff are
expected to show two distinct linear regions described by
Veff g E , f ) 1
k
Figure 1. Absolute cross-sections for ionization of nitromethane by
electron impact shown in log-log plot. Errors are estimated at (15%
for the total ionization cross-section.
1/2
Veff < E , f ) (V /E )
k
eff
k
a
TABLE 1: Comparison of Relative Ion Intensities
where Ek is the kinetic energy of the ions (a single value
isotropic kinetic energy distribution is assumed although the
+
(
Normalized to NO ) from Our FTMS Experiments and
20
from the NIST Webbook
1
7-19
method can also be applied to more complicated situations).
rel intens
NIST Webbook
The break between these two linear regions defines the kinetic
energy of the ions.
m/z
ion
FTMS
32
1
1
1
2
3
4
2
8
8
48
6
1
1
2
10
7
11
100
2
1
1
2
5
5
7
37
4
57
1
III. Results and Discussion
+
Thirteen positive ions are generated with cross-sections above
15
16
CH
3
0- cm from the electron impact ionization of nitromethane.
18
2
1
1
1
2
2
7
8
6
7
The partial cross-sections as functions of the electron energy
are shown in Figure 1. There are mass spectral data at 70 eV
compiled by the National Institute of Standards and Technology
CHN+
5
6
8
100
1
+
(
NIST) Mass Spec Data Center published in the NIST Web-
28
29
2
CH N
20
+
book, which are shown in Table 1 for comparison with our
current FTMS data. In general the two sets of data show similar
major fragmentation patterns; the relative intensities of fragment
ions are similar, except that we observe significantly less of
the parent ion than ref 20. Our data show the total ionization
CH
NO
3
N
+
30
31
32
41
CH
3
O+
+
42
43
44
CNO
1
2
13
5
28
3
28
-
16
2
+
cross-section reaches a maximum of 6.8 × 10 cm at 60 eV.
CHNO
+
+
+
CH NO
The parent ion, CH3NO2 , remains one of the more significant
2
45
46
60
61
CH
NO
3
NO
2
ions within the studied energy range of 10-200 eV, and the
most important fragment ions include NO , NO2 , CH3 , and
CH2NO . The neutral partners of interest cannot be determined
experimentally using FTMS but are briefly discussed here on
the basis of results from the literature. For NO , the neutral
partner is expected to be the radical CH3O, and the formation
of the ion/neutral pair proceeds through an isomerization of the
+
+
+
+
+
CH
CH
2
NO
NO
2
+
+
3
2
62
+
a
The electron energy is 70 eV. The ion formulas for the specific
m/z are shown only as the current authors’ suggestions. Ions with low
-
18
2
intensities (i.e., with cross-sections < 10
included in the FTMS results.
cm at 70 eV) are not
+
parent ion CH3NO2 followed by dissociation, which has been
studied by several groups in the past:2
1-25
+
Similarly, CH2NO is formed via another isomerization of the
parent ion followed by dissociation resulting in OH as the neutral
+
+
+
CH NO f [H CONO] f NO + CH O
(1)
3
2
3
3

9042 J. Phys. Chem. A, Vol. 107, No. 42, 2003
Jiao et al.
and is unlikely to have been substantially disturbed by ion/
molecule reactions because they are rather slow, as will be
discussed below.
We focus our attention on CH2NO2- because it is considered
to be the product of the first step of amine sensitization of
3
,4
nitromethane. It has been generally accepted that cleavage of
the C-N bond in nitromethane is a key step in the decomposi-
2
7-31
tion of the molecule.
However, the details of the decom-
32
-
position remain unclear. Engelke et al. suggested that CH2NO2
plays an important role in the rate-limiting step of the detonation.
Constantinou et al. and Cook et al. questioned the importance
29
30
-
of CH2NO2 and proposed different sensitization mechanisms
through a charge-transfer complex between nitromethane and
the amine, or a hydrogen-bonded complex, respectively. Politzer
3
1
-
et al. revisited the CH2NO2 hypothesis and pursued it further
by suggesting the most energetically favorable reaction involving
-
CH2NO2 :
-
-
CH NO + NH f CH NH + NO
(5)
2
2
3
3
2
2
3
Recently, Gruzdkov et al. proposed another mechanism for the
amine sensitization as outlined below:
Figure 2. Fraction of NO +
and CH
+
ions trapped in the cell vs the
2
3
+
square root of the effective trapping voltage. Data for NO
2
are
CH NO + RNH f CH NO + RNH3+
-
(6)
displayed so that f is offset by 0.2.
3
2
2
2
2
radical partner:23-25
-
•-
CH NO + CH NO f CH NO + CH NO2 (7)
2
2
3
2
3
2
2
+
+
+
CH NO f [H CdN(O)OH] f CH NO + OH (2)
•-
-
3
2
2
2
CH NO f CH + NO
2
(8)
3
2
3
On the other hand, NO2+ and CH3+ are thought to result from,
respectively, a direct bond cleavage of H3C-NO2 :
To offer more experimental evidence of the above mechanism,
Woods et al. used laser-initiated decomposition of single aerosol
+
22,23
4
particles and a time-of-flight mass spectrometer to study the
decomposition in nitromethane/amine mixtures. Only under
large amine concentration conditions did they observe CH2NO2
CH NO2+ f NO + + CH3
(3)
(4)
3
2
-
CH NO2+ f CH + + NO2
3
3
and at very low intensity; they correctly attributed the very low
intensity to the high reactivity of this anion. By comparing the
anion mass spectra in pure nitromethane and nitromethane/amine
In the mid 1950s, Kandel26 in an electron impact ionization
study of nitromethane found that the product ion CH3 had 1.1
0.2 eV kinetic energy while many other product ions including
NO2 were essentially kinetically thermal. In our FTMS
experiments we have probed the kinetic energy of the ions by
+
-
mixture they found larger intensity of the NO2 in the latter,
-
-
(
suggesting NO2 as the product of the reaction CH2NO2
+
(reactions 7 and 8). In discussing the reaction thermochemistry,
they found that the proposed reaction pathway (reactions 6-8)
was considerably endothermic but suggested that solvation
energies could greatly reduce the reaction enthalpy.4
1
6
the technique that is described in detail in a previous paper
and briefly outlined in the Experimental Section. Figure 2 shows
+
-
the trapping potential dependence of the ion intensities for NO2
We have examined the reactions involving CH2NO2 using
+
+
-7
-6
and CH3 . While the data for NO2 show no changes of the
FTMS under gas pressures in the range of ∼10 to 10 Torr
+
•-
fraction of trapped ions as Veff varies, the data for CH3 show
so that any intermediate product ions such as CH3NO2 can
a slope break at Veff ) 1.0 ( 0.2 V, which indicates a 1.0 (
be detected if they have lifetimes of a few milliseconds or
longer. To prevent interference from the trapped electrons in
the cell, we eject the electrons by dropping the trapping voltage
to ground to allow the charged particles to diffuse out of the
trapping cell. Both ions and electrons are lost during this field-
free period, but because at low ion density the Debye length is
large and the electrons diffuse much faster than the ions, the
length of the field-free duration can be adjusted so that almost
all of the electrons have left the trap while there are still
sufficient ions remaining in the cell for the reaction studies.
Among the ions generated from the electron attachment
+
+
0
.2 eV kinetic energy for CH3 and for NO2 insignificant
kinetic energy. Our FTMS results thus agree with the observa-
tion in ref 26.
We have studied the formation of anions by electron
attachment to nitromethane and by the subsequent ion/molecule
reactions. For the attachment experiments we use the secondary
electrons that are generated by irradiating nitromethane gas
molecules with primary electrons of 100 eV energy for 50 ms.
With the trapping potential set to -5 V, the secondary electrons
trapped in the cell are estimated to have energies ranging from
thermal to 3.3 eV (the actual potential depth in the cell). We
eject all anions formed during the time in which the primary
electron beam is on, and then, after 100 ms collision time to
allow the trapped electrons to collide with nitromethane gas
-
mentioned above, we select CH2NO2 for reactions by ejecting
all other ions out of the cell using large amplitude radio
frequency (rf) excitation that contains the cyclotron frequencies
-
of all of the ions except CH2NO2 . Typical reaction mass spectra
-
-
-
molecules, we observe CH2NO2 , NO2 , and CN in roughly
equal intensities. This ion population appears to result from the
attachment of the electrons with energies of thermal to 3.3 eV
are shown in Figure 3. Three product anions are observed from
-
-
-
the reaction of CH2NO2 with nitromethane: NO2 (73%), CN
-
(15%), and CNO (12%), with the branching ratios shown in

Positive and Negative Ion Formation in CH3NO2
J. Phys. Chem. A, Vol. 107, No. 42, 2003 9043
the authors’ suggestion only and has not been proved experi-
mentally or theoretically. The neutral products of the minor
-
-
reaction channels that form CN and CNO , respectively,
cannot be uniquely identified. The overall reaction rate produc-
ing these three anions is rather small. In the above anion reaction
experiments, the CH3NO2 pressure is raised 10 times higher
-
6
(
on the order of 10 Torr) than in the electron impact or
attachment experiments, so that we can measure a significant
change of the ion population within the given reaction time.
The absolute reaction rate has not been measured because the
vapor pressure of CH3NO2 is too low for an accurate pressure
calibration procedure using a pulsed valve and a spinning rotor
3
4
-
friction gauge. However, we can compare the rate of CH2NO2
+
reaction with that of Ar reaction under the same reactant
+
pressure. Ar is found to react with neutral nitromethane
+
+
+
molecule producing NO2 (41%), NO (32%), CH3NO (14%),
CH3 (10%), CH2NO (2%), and CH3N (1%), with a decay
rate approximately 80 times faster than the reaction rate of
+
+
+
-
+
CH2NO2 . Given that the Langevin collision rate for the Ar
-
9
3
reaction is calculated to be 1.3 × 10 cm /s using the
-
24
3 35
polarizability of nitromethane to be 7.37 × 10
cm , the
cm /s. Such a low
-
-11
3
NO2 reaction rate should be e1.6 × 10
-
Figure 3. Mass spectrum of the isolated CH
2
NO
2
acquired (a) without
reaction rate probably reflects a poor steric factor of reaction
3
delay after isolation and (b) with a delay of 300 ms to react with CH -
9
. The low reaction rate raises the question of the possibility
-
6
NO
2
(∼10 Torr) after isolation.
that the observed reaction is due to the 0.9% (maximum value)
of the impurity in the CH3NO2 sample. Although the impurity
concentration appears to be too small to account for the observed
reaction rate (shorter by ∼40%), this possibility cannot be
completely excluded at this point.
The measurements of the reaction kinetics have identified a
-
-
possible reaction pathway for CH2NO2 to form NO2 in the
gas phase that is thermochemically allowed. Whether this
reaction actually takes place in the process of amine sensitization
of nitromethane is not known, and further investigation into this
question is desirable. The reaction rate in the gas phase is low,
but in the condensed phase it should be quite rapid.
3
6
In the early 1980s, Domenico et al. studied negative ions
in the mass spectrum of nitromethane, and on the basis of the
pressure dependence of the ion intensity, they identified the
primary ions (ions formed by the electron attachment) and the
secondary ions (ions resulted from the ion/molecule reactions).
They suggested the following reaction produces some of the
-
CH2NO2 ions they detected:
NO2- + CH NO f CH NO + HNO2
-
3
2
2
2
Figure 4. Semilogarithmic plot of the anion intensities vs reaction
time. The solid lines are the curve fittings assuming the rate constant
However, this reaction is not observed in our experiments. We
have isolated NO2 from other ions to observe its reactivity
with the parent gas unambiguously, but even after a considerable
time delay no detectable product ions are seen.
-
1
-
-
-
as 3.1 s and the branching ratios to generate NO
as 73, 15, and 12%, respectively.
2
, CN , and CNO
-
parentheses. There are two possible explanations for the
-
observed gas-phase reaction of CH2NO2 : (a) the reaction is
IV. Summary
exothermic or (b) the reaction is a “hot ion” reaction. To probe
the possibility of the hot ion reaction, the reactant intensity as
a function of the reaction time is measured, and the resultant
kinetics plot is presented in Figure 4. The data show a single-
Electron impact ionization of nitromethane in the energy
range from threshold to 200 eV produces 13 positive ions, with
+
+
+
+
+
CH NO , CH NO , NO , NO , and CH as the most
3
2
3
2
3
-
exponential behavior of the CH2NO2 decay, suggesting it is
abundant ions. The total ionization cross-section peaks at 60
-
16
2
an exothermic reaction rather than a hot ion reaction. On the
eV with the value of 6.8 × 10
cm . The estimated error of
-
+
basis of this result we propose the reaction of CH2NO2 forming
the partial ionization cross-sections is (15%. CH is found to
3
-
NO2 as
have 1.0 ( 0.2 eV kinetic energy, while other fragment ions
have insignificant kinetic energies, in a good agreement with
-
-
2
2
6
CH NO + CH NO f NO + CH CH NO
2
(9)
the findings reported in the literature.
2
2
3
2
3
2
Dissociative electron attachment of nitromethane in the energy
because it has a reaction enthalpy of -102 kJ/mol.33 The
composition of the neutral product in the above equation presents
range of 0-3.3 eV is found to produce three detectable anions,
-
-
-
-
CH2NO2 , NO2 , and CN . CH2NO2 is observed to react with

9044 J. Phys. Chem. A, Vol. 107, No. 42, 2003
Jiao et al.
-
the parent molecule to form NO2 . We find the following: (1)
(8) Liang, Z.; Marshall, A. G. Anal. Chem. 1990, 62, 70.
(9) Marshall, A. G.; Wang, T. L.; Ricca, T. L. J. Am. Chem. Soc. 1985,
07, 7983.
10) Guan, S. J. Chem. Phys. 1989, 91, 775.
(11) Chen, L.; Marshall, A. G. Int. J. Mass Spectrom. Ion Processes
1987, 79, 115.
•-
at low pressures no intermediate product ion such as CH3NO2
is formed (CH2NO2 produces NO2 directly, along with minor
product ions CN and CNO ); (2) the reaction is exothermic
1
-
-
(
-
-
-
11
3
and yet slow, with the rate e1.6 × 10
cm /s; (3) no other
(12) Rempel, D. L.; Huang, S. K.; Gross, M. L. Int. J. Mass Spectrom.
anions are found to be reactive with the parent molecule. Due
to the time resolution of the FTMS, observation 1 does not rule
Ion Processes 1986, 70, 163.
(
13) Straub, H. C.; Renault, P.; Lindsay, B. G.; Smith, K. A., Stebbings,
R. F. Phys. ReV. A 1995, 52, 1115.
14) Guan, S.; Marshall, A. G. Int. J. Mass Spectrom. Ion Processes
995, 146/147, 261.
15) Wang, M.; Marshall, A. G. Anal. Chem. 1989, 61, 1288.
•-
out the possibility of CH3NO2 as an intermediate product that
undergoes unimolecular dissociation with a lifetime shorter than
milliseconds. If this was the case, we could not observe such
an intermediate. Referring to the previously proposed amine
sensitization mechanism of nitromethane, which includes an
endothermic reaction pathway from CH2NO2 to form NO2 ,
the current FTMS results point out another possible reaction
pathway (reaction 9) which is exothermic. Whether this reaction
pathway makes a significant contribution to amine sensitization
in the condensed phase is a question which warrants further
investigation.
(
1
2
(
(16) Jiao, C. Q.; DeJoseph, C. A., Jr.; Garscadden, A. J. Chem. Phys.
002, 117, 161.
(
17) Gord, J. R.; Freiser, B. S. J. Chem. Phys. 1991, 94, 4282.
-
- 3,4
(18) Rincon, M. E.; Pearson, J.; Bowers, M. T. J. Phys. Chem. 1988,
9
2, 4290.
(19) Derai, R.; Mauclaire, G.; Marx, R. Chem. Phys. Lett. 1982, 86,
2
75.
(20) http://webbook.nist.gov/.
(21) Baer, T.; Hass, J. R. J. Phys. Chem. 1986, 90, 451.
(22) McKee, M. L. J. Phys. Chem. 1986, 90, 2335.
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Ion Processes 1988, 84, 271.
25) Sirois, M.; Homes, J. L.; Hop, C. E. C. A. Org. Mass Spectrom.
(
1
Acknowledgment. The authors thank the Air Force Office
of Scientific Research for support.
(
(
Note Added in Proof. W. Sailer et al.37 have reported
recently on absolute partial cross sections for dissociative
electron attachment (DEA) to nitromethane. They used a crossed
electron beam-molecular beam interacting system whose
products were measured using a quadrupole mass spectrometer.
The data as a function of electron energy over the range 0 to
.5 eV show various resonances. They explain the detection of
and CNO at low electron energies in terms of DEA to
multimode energy transfer from vibrationally excited molecules.
1
990, 25, 167.
(26) Kandel, R. J. Chem. Phys. 1955, 23, 84.
(27) Constantinou, C. P. The Nitromethane-Amine Interaction. Ph.D.
dissertation, Cambridge University, Cambridge, U.K., 1992.
28) Gupta, Y. M.; Pangilinan, G. I.; Winey, J. M.; Constantinou, C. P.
(
Chem. Phys. Lett. 1995, 232, 341.
(29) Constantinou, C. P.; Mukundan, T.; Chaudhri, M. M. Philos. Trans.
R. Soc. London A 1992, 339, 403.
9
O
(
30) Cook, M. D.; Haskins, P. J. Proc. 9th Symp. (Int.) Detonation 1989,
027. Cook, M. D.; Haskins, P. J. Proc. 10th Symp. (Int.) Detonation 1993,
870.
-
-
1
(31) Politzer, P.; Seminario, J. M.; Zacarias, A. G. Mol. Phys. 1996,
8
9, 1511.
(
References and Notes
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