Mass-spectrometric experiments together with electronic structure calculations support the existence of the elusive ammonia oxide molecule and its radical cation

Article abstract of DOI:10.1002/(SICI)1099-0682(199810)1998:10<1529::AID-EJIC1529>3.0.CO;2-Y

Mass-spectrometric experiments were combined with ab initio calculations to explore the cationic and neutral [H₃,N,O]^?+/0 potential energy surfaces and relevant anionic species. The calculations predict the existence of three stable cationic and neutral [H₃,N,O]^?+/0 isomers, i.e. ammonia oxide H₃NO^?+/0 (1^?+/0), hydroxylarnine H₂NOH^?+/0 (2^?+/0) and the imine-water complex HNOH₂^?+/0 (3^?+/0). Hydroxylamine 2 represents the most stable isomer on the neutral surface (E_rel = 0), and the metastable isomers 1 (E_rel = 24.8 kcal mol^-1) and 3 (E_rel = 61.4 kcal mol^-1) are separated by barriers of 49.5 kcal mol^-1 and 64.2 kcal mol^-1, respectively. Adiabatic ionization of 2 (IE_a = 9.15 eV) yields 2^?+, which is 21.4 kcal mol^-1 more stable than 1^?+ and 36.4 kcal mol^-1 more stable than 3^?+. The barriers associated with the isomerizations of the cations are 58.6 kcal mol^-1 for 2^?+ → 1^?+ and 71.4 kcal mol^-1 for 2^?+ → 3^?+. Collisional activation (CA) and unimolecular decomposition (MI) experiments allow for a clear distinction of 1^?+ from 2^?+. Besides, neutralization/reionization (NR) experiments strongly support the gas-phase existence of the long-sought neutral ammonia oxide.

Full text of DOI:10.1002/(SICI)1099-0682(199810)1998:10<1529::AID-EJIC1529>3.0.CO;2-Y

FULL PAPER
Mass-Spectrometric Experiments together with Electronic Structure
Calculations Support the Existence of the Elusive Ammonia Oxide Molecule
Ƞ
and Its Radical Cation
Mark Brönstrup, Detlef Schröder, Ilona Kretzschmar, Christoph A. Schalley[°], and Helmut Schwarz*
Institut für Organische Chemie der Technischen Universität Berlin,
Straße des 17. Juni 135, D-10623 Berlin
Fax: (internat.) ϩ 49(0)30/314-21102
E-mail: schw0531@www.chm.tu-berlin.de
Received May 12, 1998
Keywords: Ab initio calculations / Mass spectrometry / Gas-phase chemistry / Ammonia oxide / Hydroxylamine
Mass-spectrometric experiments were combined with ab
initio calculations to explore the cationic and neutral
49.5 kcal mol–1 and 64.2 kcal mol–1, respectively. Adiabatic
ionization of 2 (IE
a
= 9.15 eV) yields 2•+, which is 21.4 kcal
[H
3
,N,O]•+/0 potential energy surfaces and relevant anionic
mol more stable than 1•+ and 36.4 kcal mol–1 more stable
–1
species. The calculations predict the existence of three stable
than 3•+. The barriers associated with the isomerizations of
cationic and neutral [H
3
,N,O]• isomers, i.e. ammonia oxide
+/0
the cations are 58.6 kcal mol for 2 Ǟ 1 and 71.4 kcal
–1
•+
•+
•
+/0
•+/0
•+/0
•+/0
–1
•+
•+
H
3
NO
(1
), hydroxylamine H
2
NOH
•+/0
(2
) and the
mol
for
2
Ǟ 3 . Collisional activation (CA) and
•
+/0
imine-water complex HNOH
2
(3
). Hydroxylamine 2
unimolecular decomposition (MI) experiments allow for a
clear distinction of 1•+ from 2 . Besides, neutralization/
•+
represents the most stable isomer on the neutral surface
–1
(Erel = 0), and the metastable isomers 1 (Erel = 24.8 kcal mol ) reionization (NR) experiments strongly support the gas-
and 3 (Erel = 61.4 kcal mol–1) are separated by barriers of
phase existence of the long-sought neutral ammonia oxide.
Introduction
Nowadays, theoretical calculations of small systems, like
[
H ,N,O], can achieve an accuracy such that they do comp-
The generation of ammonia oxide (1) represents a major
challenge to experimentalists working in the field of elusive
molecules, as there is a striking discrepancy between the
3
lement the experimental results. Therefore, we have per-
formed a series of mass-spectrometric experiments and cal-
culated all relevant structures of the cationic and neutral
[1]
numerous quantum chemical studies, which predict a dis-
tinct stability of the molecule, and the still scarce exper-
[H ,N,O] surfaces with a G2-like ab initio method; also the
3
[2]
anionic species are briefly discussed (see below). The neu-
imental evidence. Moreover, it has recently been stated
that “the possible characterization of two metastable iso-
mers of hydroxylamine ... would be a significant step for-
ward towards the understanding of important aspects of N/
tral surface was reexamined using the same level of theory,
as previous theoretical studies did not include the relevant
exit channels. For the sake of simplicity and readability, the
potential energy surfaces (PES) of the different charge
states are calibrated to the respective global minima, which
are set to zero (Erel ϭ 0). As all the experiments start from
cationic states, the cationic PES will be discussed first.
[2a]
O/H chemistry.”
The fact that homologs of 1 like trialkylamine oxides are
well-known, storable compounds indicates that the atom
connectivity R NϪO is not inherently unstable. Therefore,
3
for R ϭ H the main problem is to prevent the isomerization
of 1 to the more stable hydroxylamine (2). This can occur
in two ways, i.e. either a unimolecular [1,2]-H shift or a
bi- or termolecular acid/base-catalyzed path, of which we
presume the latter to be energetically favorable. Such an [H ,N,O] surface are displayed in Table 1. Ionized hy-
acid/base-catalyzed isomerization can be circumvented by droxylamine (2 ) represents the global minimum (Erel
Theoretical Results
The relative energies of all relevant species of the
•
ϩ
3
•
؉
ϭ
generating the species of interest in the diluted gas phase of 0) and possesses a planar structure. Mulliken analysis re-
a mass spectrometer. The generation and characterization veals that the spin density is distributed between the nitro-
of several elusive ionic and neutral molecules has been gen (0.80) and the oxygen atom (0.20). This indicates that
achieved during the last decade by combining advanced the unpaired electron is mainly located on the nitrogen
mass-spectrometric techniques, like collisional activation atom, but that there is also exchange with the adjacent oxy-
(
CA) and neutralization/reionization (NR), with appropri- gen lone pairs. As a consequence, the NϪO bond length is
[3][4]
ate theoretical means.
significantly shortened compared to neutral hydroxylamine
128 pm vs. 144 pm, see below). A [1,2]-H shift from the
nitrogen to the oxygen atom requires an energy of 58.6 kcal
(
[°]
Present address: The Skaggs Institute for Chemical Biology, The
Scripps Research Institute, 10550 North Torrey Pines Road, La
Jolla, CA, 92037, USA.
Ϫ1
•؉
mol and leads to the ammonia oxide cation (1 ), which
Eur. J. Inorg. Chem. 1998, 1529Ϫ1538

WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
1434Ϫ1948/98/1010Ϫ1529 $ 17.50ϩ.50/0
1529

M. Brönstrup, D. Schröder, I. Kretzschmar, C. A. Schalley, H. Schwarz
FULL PAPER
is a minimum with Erel ϭ 21.4 kcal molϪ1. Thus, the calcu- Table 1. Total energies E in Hartree for [H
, N, O]•ϩ/0 species; E ϭ
E[CCSD(T)/6-311G(d,p)] ϩ E[MP2/6-311ϩG(3df,2p)] Ϫ E[MP2/
-311G(d,p)] ϩ ZPVE
3
•
؉
lations suggest that the ammonia oxide cation (1 ) is
trapped in a potential well which may be deep enough to
capture 1• without facile isomerization to 2•؉ or dis-
sociation. With respect to the lowest lying direct exit chan-
6
؉
Species
Total energy in Hartree
•
ϩ
•
nel for the dissociation of [H ,N,O] , i.e. loss of H (E
ϭ
NH O (1)
Ϫ131.453218
Ϫ131.403453
Ϫ131.451865
Ϫ131.492710
Ϫ131.399248
Ϫ131.486162
Ϫ131.444508
Ϫ131.394834
Ϫ131.387307
Ϫ131.413806
Ϫ131.390428
Ϫ131.314067
Ϫ131.351575
Ϫ131.375234
Ϫ131.119145
Ϫ131.118677
Ϫ131.156460
Ϫ131.098877
Ϫ131.098450
Ϫ131.063322
Ϫ131.001773
Ϫ131.013807
Ϫ131.042749
Ϫ131.009346
Ϫ131.001773
Ϫ131.015157
3
rel
•؉
3
3
3
9.7 kcal mol ), unimolecular isomerization 1•؉ v 2 is,
Ϫ1
3
NH O ( 1)
6
•
؉
NH
3
O, with geometry of 1
•
؉
however, conceivable. The ammonia oxide cation (1 ) has
NH
2
OH (2)
2
3
3
C symmetry due to a small Jahn-Teller distortion of the
NH
OH ( 2)
s
NH
NH
2
OH, gauche,gauche conf.
OH, with geometry of 2
(3)
C -symmetric molecule. The unpaired electron is located
3v
•؉
2
on the oxygen atom (spin density ϭ 1.05). In contrast to
NHOH
2
1
3
3 3
hydroxylamine, the NϪO bond is not shortened, but MECP NH O/ NH O
TS 1/2
slightly elongated upon ionization of the neutral 1 (138 pm
TS 2/3
vs. 135 pm, see below); this can be explained in terms of a TS 2/6
TS 2/7
diminished electrostatic interaction compared to the neutral
TS 1/6
ϩ
Ϫ
H N ϪO zwitterion. Another minimum structure of
•ϩ
NH O (1•؉
3
)
3
•
ϩ
•ϩ
[
H ,N,O] is obtained by a [1,2]-H shift from the nitrogen NH
3
O
2
2
, with geometry of 1
3
•ϩ •؉
NH
NH
OH (2 )
to the oxygen atom starting from hydroxylamine. The re-
ϩ
OH• , with geometry of 2
•
؉
Ϫ1
sulting distonic ion 3 (Erel ϭ 36.4 kcal mol ) can be
•ϩ
•؉
NHH O (3
2
)
regarded as an ion-dipole complex between the imine frag- TS 1• /2
؉
•؉
TS 2• /6
؉
•؉
ment (IE ϭ 13.49 eV) and the water cation (IE ϭ 12.612
•؉ •؉
TS 1 /6
•؉ •؉
[
5]
eV). The barrier associated with this [1,2]-H shift (Erel
ϭ
•ϩ
TS 2• /6
؉ •؉
1.4 kcal molϪ1) implies that the NH OH•ϩ/NHOH2
isomerization is disfavored compared to the NH OH
TS 2 /7
؉ •؉
7
2
TS 5 /6
•؉
•
ϩ
/
؉
2
TS 4 /6
•
ϩ
NH O equilibrium. Moreover, the exit channel for loss of
3
•
ϩ
ϩ
H to yield H NO
4 is lower in energy (Erel ϭ 69.7 kcal
2
Ϫ1
ϩ
؉
mol ) and has much smaller entropical restrictions than correspond to an attack of a hydrogen atom at H NO (4 )
2
•
؉
•؉
ϩ
؉
•ϩ
•؉
the rearrangement 2 Ǟ 3 . Therefore, we expect that and at HNOH (5 ), respectively, to yield HNO (6
NHOH2• (3 ), can hardly be generated by isomerization and H . Although the barriers associated with these tran-
starting from 1 and 2 when the [H ,N,O] surface is sition structures are comparable to those for TS 1 /6
)
ϩ
•؉
2
•
؉
•؉
•ϩ
•؉ •؉
3
•
؉
•؉
؉
•؉
accessed experimentally. A further proton shift from the ni- and TS 2 /7 , it is hard to imagine that TS 4 /6 or TS
؉
•؉
trogen to the oxygen atom and consecutive fragmentation 5 /6 can be reached, because instead of a long-range H
ϩ
4
•؉
•؉
yields H O and N ( S) as the lowest lying exit channel of migration starting from 1 or 2 , an irreversible loss of
3
Ϫ1
the cationic PES (E ϭ 49.3 kcal mol ). However, as the migrating hydrogen atom appears to be favored. There-
H O and N ( S) can only be produced after spin crossing fore, it is suggested that the only ways to produce HNO
rel
ϩ
4
•ϩ
/
/
3
•
ϩ
•؉
and extensive isomerization, severe kinetic restrictions are H from [H ,N,O] proceed by 1,1-eliminations (TS 1
imposed on this otherwise thermochemically favorable exit
channel.
2
؉
3
•
•؉ •؉
ϩ
6
and TS 2 /7 ). Two different pathways of NO gen-
•
ϩ
eration from [H ,N,O] are conceivable: Either H elimin-
3
2
•
ϩ
•
•
Cleavage of one of the NϪH bonds in ionized hy- ation to yield [H,N,O] precedes H abstraction, or H is
droxylamine (2• ) gives H and cis- or trans-HNOH (5 ) lost first to yield 4 and/or 5 , which can subsequently be
؉
•
ϩ
؉
؉
؉
Ϫ1
Ϫ1
ϩ
with Erel ϭ 87.0 kcal mol (trans) and 93.6 kcal mol
dehydrogenated to NO . The calculations predict that the
[
6]
•؉
•؉
(
[
cis), respectively (Table 2).
Further exit channels of initial H2 eliminations either from 1 or from 2 are
H ,N,O] yield HNO (6•؉) and H2 (Erel ϭ 58.4 kcal strongly favored by at least 66.1 kcal mol compared to
•
ϩ
•ϩ
Ϫ1
3
Ϫ1
•ϩ
•؉
Ϫ1
؉
؉
mol ), NOH (7 ) and H (E ϭ 73.2 kcal mol ), or the dehydrogenations of 4 and 5 (Figure 1). Therefore, it
2
rel
ϩ
؉
•
Ϫ1
NO , 8 , H , and H (E ϭ 77 kcal mol ). Although can be concluded that only the pathway involving loss of
2
rel
•
ϩ
these exit channels are low in energy, they are associated H followed by loss of H is relevant for NO production
2
with significant activation barriers. Thus, the transition in the present context.
state TS 1• /6 for 1,1-elimination of H from 1 has a
؉
•؉
•؉
Losses of one or several hydrogen atoms do not reveal
2
Ϫ1
•ϩ
relative energy of 89.5 kcal mol , and the transition state information about the connectivity of the [H ,N,O] pre-
3
•
؉
•؉
TS 2 /7 for the analogous 1,1-elimination of H2 from cursors. For a structural elucidation, fragments arising from
•
؉
Ϫ1
2
has a relative energy of 92.3 kcal mol . Despite an cleavage of the NϪO bond are decisive: Cleavage of NϪO
intensive search from several starting geometries, a tran- in 2 into NH2 ( B ) and OH has an energy demand of
sition state for a formal 1,2-elimination from 2 could not Erel ϭ 104.0 kcal mol , whereas the NϪO cleavage of 1
be located. Instead, optimization leads to two transition to yield O ( P) and NH
structures TS 5 /6 and TS 4 /6 , which are featured For 2 , the calculations predict a competition of two pro-
by linear OϪHϪH (TS 5 /6 ) and NϪHϪH (TS 4 /6
•
؉
ϩ
3
•
1
Ϫ1
•
؉
•؉
3
•ϩ
requires Erel ϭ 79.4 kcal molϪ1.
3
؉
•؉
؉
•؉
•؉
؉
•؉
؉
•؉
ϩ
3
•
)
cesses: The direct bond cleavage into NH2 ( B ) and OH
1
arrangements. An analysis of the internal reaction coordi- is energetically disfavored, but has probably no kinetic re-
ϩ
nate (IRC) reveals that the respective reaction pathways striction. In contrast, while the production of NH3• ϩ O
1530
Eur. J. Inorg. Chem. 1998, 1529Ϫ1538

Existence of the Elusive Ammonia Oxide Molecule and Its Radical Cation
FULL PAPER
Table 2. Total energies E in Hartree for [H
products; E ϭ E[CCSD(T)/6-311G(d,p)) ϩ E(MP2/6-311ϩG-
3df,2p)] Ϫ E[MP2/6-311G(d,p)] ϩ ZPVE
3
,N,O]•ϩ/0 fragmentation and NH3•ϩ is favored due to thermochemical and kinetic
reasons.
(
The profile of the neutral [H ,N,O] surface resembles its
cationic counterpart as far as the hydroxylamine/ammonia
3
Species
Total energy in Hartree
oxide region is concerned. Hydroxylamine (2) again rep-
•
•
H
2
NO (4 )
Ϫ130.871158
Ϫ130.860474
Ϫ130.852498
Ϫ130.282162
Ϫ130.237766
Ϫ129.701610
Ϫ56.435131
Ϫ74.966971
Ϫ75.625497
Ϫ55.770735
Ϫ1.160442
•
•
resents the global minimum and has two C -symmetrical
s
trans-HNOH (5 )
•
•
Ϫ1
cis-HNOH (5 )
HNO (6)
conformers which differ in energy by 4.1 kcal mol (Erel
ϭ
ϭ
Ϫ1
3
0 for θHNOH ϭ 125.1°, Erel ϭ 4.1 kcal mol for θHNOH
NOH (7)
NO (8 )
•
•
57.4°). Hydroxylamine (2) can isomerize to ammonia oxide
1) by a [1,2]-H shift with an energy barrier of 49.5 kcal
NH
3
(
3
O •
Ϫ1
Ϫ1
mol . 1 lies 24.8 kcal mol higher in energy than 2, but
OH
H
H
H
2
N•
may be captured as a metastable molecule due to the reverse
Ϫ1
2
barrier of 24.7 kcal mol towards isomerization to 2 and
Ϫ0.499810
Ϫ1
4
a barrier of 48.9 kcal mol associated with fragmentation
N
Ϫ54.510044
Ϫ55.126862
Ϫ55.060045
Ϫ76.308577
Ϫ130.545642
Ϫ130.517986
Ϫ130.507510
Ϫ129.902885
Ϫ129.879370
Ϫ129.373546
Ϫ130.404296
Ϫ130.383696
Ϫ56.062959
Ϫ55.316852
Ϫ55.365182
Ϫ75.847343
Ϫ54.636206
Ϫ76.56788
3
1
NH
NH
into HNO and H . The NϪO bond in 1 is significantly
2
shorter than in 2 (135 pm vs. 144 pm), which can be ratio-
nalized with an electrostatic attraction of the zwitterionic 1
and the repulsion of lone pairs in 2. The two molecules are
predicted to differ also in other physical properties such as
H
2
O
ϩ
؉
NH
2
O (4 )
ϩ
؉
trans-NHOH (5 )
ϩ
؉
cis-NHOH (5 )
•
ϩ
•؉
•؉
NHO (6
)
)
•
ϩ
[7]
NOH (7
dipole moments (µ ϭ 5.63 D for 1 and 0.788 D for 2) or
ϩ
؉
NO (8 )
[8][9]
vertical ionization energies
(IEv ϭ 9.1 eV for 1 and
؉
؉
؉
TS 4 /8
؉
10.72 eV for 2). Two factors are mainly responsible for the
TS 5 /8
•
ϩ
ϩ
Ϫ1
NH
3
fact that vertically ionized hydroxylamine is 36.1 kcal mol
1
NH
NH
2
3
ϩ
above the minimum geometry of the cation. First, the mini-
2
•
ϩ
•؉
H
2
O
mum-energy conformation of 2 is planar, while C -sym-
s
•
ϩ
ϩ
NH
metrical, neutral 2 has a dihedral HNOH angle of 125°.
3
H O
•
؉
Secondly, the NϪO bond lengths in 2 and 2 differ signifi-
cantly (Figures 3 and 4). In contrast, the geometry changes
upon adiabatic ionization of 1 are negligible, and IE and
a
ϩ
3 2
Figure 1. Potential energy surface of cationic [H )
,N,O]• ; zero-point corrected energies given in kcal molϪ1 relative to H NOH•ϩ (2•ϩ
is lower in energy, it is kinetically hampered due to the en- IE differ by no more than 0.29 kcal molϪ1. As a result, the
v
•
؉
•؉
tropic restrictions associated with TS 1 /2 . For ionized vertical ionization energies of hydroxylamine (IE ϭ 10.72
v
ammonia oxide, however, the direct fragmentation into O eV) and ammonia oxide (IE ϭ 9.1 eV) differ by as much
v
Eur. J. Inorg. Chem. 1998, 1529Ϫ1538
1531

M. Brönstrup, D. Schröder, I. Kretzschmar, C. A. Schalley, H. Schwarz
,N,O]; zero-point corrected energies given in kcal molϪ1 relative to H
NOH (2)
FULL PAPER
Figure 2. Potential energy surface of neutral [H
3
2
as 1.6 eV, although IE is comparable for both isomers.[
8][9]
a
A similar difference can be discerned for the recombination
processes of an electron with the cations to the neutral mol- Figure 3. Calculated bond lengths in pm and angles in degrees of
ϩ
,N,O]• species; for geometries of [H
ϩ
relevant cationic [H
3
2
,N,O]
ecules. In fact, these pronounced differences in physical
properties may enable an experimental distinction of these
two isomers.
[6]
species pertinent to the present work, see ref.
The stability of the third neutral isomer, the imine-water
complex 3, has very much decreased compared to the cat-
ionic surface. The minimum structure is hardly more stable
than the transition state of the [1,2]-H shift TS 2/3, and a
Ϫ1
further internal energy of 16.5 kcal mol is sufficient to
1
dissociate the molecule into NH and H O. Dissociation
2
3
Ϫ1
into NH and H O is exothermic by 25.5 kcal mol , but a
2
1
spin-forbidden process for 3.
The relative heights of the exit channels are also different
from the cationic surface, especially as far as the structure-
indicative channels involving NϪO bond cleavage are con-
•
cerned. In this respect, dissociation of [H ,N,O] into H N
3
2
•
and OH represents the lowest lying, barrier-free exit chan-
nel with a relative energy of 60.5 kcal mol , whereas the
spin-allowed dissociation into O ( D) and NH becomes
Ϫ1
1
3
quite unfavorable with a relative energy of 102.2 kcal
Ϫ1
mol . Therefore, the dissociation behaviour of the neutral
isomers is expected to be quite different from that of the
•
•
cations, in that for [H ,N,O] formation of H N and OH
3
2
ϩ
•
should be more pronounced than NH
ϩ OH for
2
•
ϩ
[
H ,N,O] . The relative stability of the closed-shell pair
HNO ϩ H (31.4 kcal mol ) is much higher than H losses
3
Ϫ1
•
2
•
•
to yield the open-shell pairs H NO ϩ H (76.4 kcal
2
Ϫ1
•
•
Ϫ1
mol ), trans-HNOH ϩ H (83.1 kcal mol ) and cis-
HNOH• ϩ H• (89.6 kcal molϪ1). However, HNO ϩ H2
formation is again hampered by significant activation bar-
Ϫ1
riers of 73.7 kcal mol for 1,1-elimination from H NO and
3
Ϫ1
of 112.1 kcal mol for 1,2-elimination from H NOH. Be-
sides, NOH ( AЈЈ) and H (E ϭ 59.3 kcal mol ) are ob-
2
3
Ϫ1
2
rel
tained by 1,1-elimination of H from H NOH, which is as-
2
2
Ϫ1
sociated with a barrier of 88.6 kcal mol
.
1532
Eur. J. Inorg. Chem. 1998, 1529Ϫ1538

Existence of the Elusive Ammonia Oxide Molecule and Its Radical Cation
Figure 4. Calculated bond lengths in pm and angles in degrees of
FULL PAPER
•
Ϫ
As far as [H ,N,O] anions are concerned, three mini-
3
3
relevant neutral [H ,N,O] species
mum structures could be located that all correspond to
weakly bound ion-dipole complexes.[
relative energies are E ϭ 0 for HNH ···OH , E ϭ 3.1
kcal mol for HOH···NH and Erel ϭ 7.7 kcal mol for
4][11]
The respective
•
Ϫ
rel
rel
Ϫ1
•Ϫ
Ϫ1
•
Ϫ
H NH···O . None of the complexes possesses an intact
2
•
Ϫ
NϪO bond, and dissociations into H N and OH (E
ϭ
rel
2
5.3 kcal mol ), NH and O (Erel ϭ 23.3 kcal molϪ1) or
Ϫ1
•Ϫ
1
3
•
Ϫ
Ϫ1
into HN
and H O (E ϭ 27.3 kcal mol ) can occur
2 rel
easily. We would like to point out that both neutral isomers
and 2 possess negative vertical electron affinities, i.e.
1
Ϫ1
Ϫ1
EA ϭ Ϫ22.8 kcal mol for 1 and EA ϭ Ϫ47.4 kcal mol
v
v
for 2. Thus, theory strongly suggests that a distinction of
neutral 1 and 2 by electron attachment experiments, which
have been applied successfully to the generation and identi-
[4][11]
fication of the neutral singlet water oxide molecule,
is
not feasible for the [H ,N,O] system.
3
In conclusion, the quantum chemical calculations predict
•
ϩ/0
the existence of three [H ,N,O]
isomers. Whereas the
3
shapes of the neutral and cationic PES are similar concern-
ing the hydroxylamine/ammonia oxide tautomerism, dis-
tinct differences exist with respect to the energetics of the
fragmentation channels.
Experimental Results
The analysis of the hydroxylamine/ammonia oxide sys-
tem based on dissociation products is hampered by an ana-
lytical problem, because the structure-indicative cleavages
of the NϪO bonds lead to fragments of equal masses, i.e.
•
•
NH2 and O (both 16 amu) as well as NH and OH (both
3
1
7 amu). Therefore, we decided to carry out additional
experiments with the fully deuterated analogs [D ,N,O],
The NH ϩ O ( P) exit channel has a comparatively low such that the combination of the two sets of data allows for
3
3
3
Ϫ1
relative energy of 56.8 kcal mol , but cannot be entered a deconvolution of the overlapping fragments, i.e. ND and
2
from the singlet surface in a spin-allowed manner. However, OD (both 18 amu) as well as ND and D O (both 20 amu).
3
2
3
•؉
•؉
two processes that eventually yield O ϩ NH should be In order to produce the three different isomers 1 , 2
mentioned briefly: (i) The dissociation of H NO ( 1), and 3 , the following precursors were chosen. (i) A mix-
which has an energy of 56 kcal mol relative to 2 and 31.2 ture of NH and N O was subjected to chemical ionization
kcal mol relative to NH O ( 1), is spin-allowed. How- in which one may envision N O as a donor for atomic oxy-
ever, the NϪO distance in geometry-optimized 1 is 296 gen to afford ammonia oxide cation 1 . (ii) Pure hy-
pm, revealing that the structure actually corresponds to a droxylamine was ionized to (predominantly) yield 2• . (iii)
O···NH van der Waals complex with a binding energy of Surprisingly, electron ionization of an aqueous N H solu-
only 0.8 kcal mol . A triplet structure for H NOH could tion also yields reasonable amounts of [H ,N,O] , and
,
3
3
3
•؉
3
Ϫ1
3
2
Ϫ1
1
1
3
2
3
•؉
؉
3
3
3
•ϩ
Ϫ1
2
3
not be located, but a second minimum on the triplet surface high mass-resolution and labeling experiments assured that
•
•
corresponding to H N ···HϪO with r
ϭ 200 pm was the selected beam actually had the desired elemental com-
2
NϪH
Ϫ1
1
•ϩ
found with an energy of 58.7 kcal mol relative to 2, only position. The mechanism of [H ,N,O] formation could
.9 kcal mol below the separated species H N and HO . not be established and seems particularly obscure for N H/
2 3
ii) In addition, the NH ϩ O ( P) exit channel might be H O. Although we cannot exclude rigorously ion-molecule
accessed by spin crossing of 1 en route to oxygen dis- reactions, the fact that the highest yields of [H ,N,O] are
3
Ϫ1
•
•
1
3
3 2
(
1
•ϩ
3
sociation. The highest spin crossing probability is achieved obtained under electron ionization conditions suggests that
at geometries where both spin states possess the same en- a volatile, hitherto uncharacterized N,H,O compound is lib-
ergy. Therefore, the minimum energy crossing point erated from an aqueous mixture of NaN3 and H SO .
2
4
(
MECP) for the H NϪO dissociation was located with a Methods (i) and (iii) gave similar spectra (Table 3), but the
3
[10]
•ϩ
recently developed method.
Most efficient crossing is ex- [H ,N,O] intensities obtained from N H/H O were much
3 3 2
pected at NϪO distances of 192 pm, where the molecule higher than those from NH /N O. Thus, in the following
3
2
Ϫ1
has an energy of 66.1 kcal mol relative to 1.
discussion, the fragmentation patterns obtained for ionized
Eur. J. Inorg. Chem. 1998, 1529Ϫ1538
1533

M. Brönstrup, D. Schröder, I. Kretzschmar, C. A. Schalley, H. Schwarz
FULL PAPER
NH OH are compared with those for N H/H O and their mentations. In addition, a fraction of “hot” molecules pos-
2
3
2
deuterated analogues.
sesses lifetimes long enough to allow for extensive re-
3 3
Table 3. Relative intensities of fragments generated from different [H ,N,O] and [D ,N,O] precursors
[
H
3
,N,O]
Fragment mass in amu
Experiment
Precursors
33
32
31
30
19
18
17
16
15
14
NH
NH
HN
2
3
3
OH
2
100
100
100
20
5
6
2
1
0.4
2
4
0.5
MI
/N
/H
O
O
2
NH
NH
HN
2
3
3
OH
2
100
100
100
18
12
14
27
16
19
0.4
0.4
1
1
1
2
3
5
11
4
3
4
0.6
1
1
0.2
0.2
0.2
CA
/N
/H
O
O
2
NH
NH
HN
2
3
3
OH
2
100
100
100
71
48
62
12
14
16
20
17
21
1
11
15
17
16
3
1
7
1
NR
/N
/H
O
O
19
22
2
1
2
2
2
2
NR-CA
NH
HN
2
3
OH
/H
100
100
24
30
38
40
9
30
13
20
2
2
O
[
D
3
,N,O]
Fragment mass in amu
Experiment
Precursors
36
34
32
30
1
22
20
18
0.6
16
14
ND
ND
DN
2
3
3
OD
2
100
100
100
2
1
0.5
4
12
MI
/N
/D
O
O
2
ND
ND
DN
2
3
3
OD
2
100
100
100
15
10
15
18
13
14
1
1
1
7
14
15
10
9
8
1
2
1
0.5
1
0.2
CA
/N
/D
O
O
2
ND
ND
DN
2
3
3
OD
2
100
100
100
56
31
56
9
5
17
10
10
18
7
6
15
27
15
51
5
2
17
1
NR
/N
/D
O
O
2
2
NR-CA
ND
2
OD
100
26
34
18
25
4
The intensities of the metastable ion (MI) spectra are low, arrangements preceding decomposition, which is indicated
•
ϩ
ϩ
•ϩ
ϩ
consistent with the fact that [H ,N,O] has only few degrees by fragments like H O and H O (or D O and D O ,
3
2
3
2
3
of freedom for the storage of internal energy. In all spectra, respectively). Nonetheless, collisional activation with ions
•
the by far dominant process corresponds to H loss to yield having keV energies promotes characteristic direct bond
ϩ
[
H ,N,O] , which is the lowest lying barrier-free exit chan- cleavages which are valuable for a structural characteri-
2
nel of the system. However, as already mentioned neither zation.
this nor the other hydrogen loss channels are indicative as
Due to collision-induced isomerization, the differences in
far as structural aspects are concerned. Thus, the following the CA spectra of the two precursors are less pronounced
discussion is primarily based on the structurally indicative compared to the metastable ion decay patterns. However,
signals due to NϪO bond cleavage. Unimolecular dis- whereas ionized hydroxylamine shows an m/z 16/17 ratio of
sociation of ionized hydroxylamine yields fragments of 1.3:1, this ratio amounts to 1:2.7 for the ion observed from
masses 16 and 17 in a 1.3:1 ratio, while only mass 17 is the HN /H O mixture (Figures 5a and 5b). Analogous re-
3
2
observed in the MI experiment using the HN /H O mixture sults are obtained with the deuterated compounds, in that
3
2
•
ϩ
as precursor. Accordingly, the MI spectrum of [D ,N,O]
the 18/20 ratio for ND OD is 1.4:1, as compared to 1:1.9
2
3
obtained from DN /D O gives rise to a single signal at mass for the DN /D O mixture. In order to probe whether an
3
2
3
2
•
ϩ
2
0, which demonstrates that exclusively ND3 is formed. alteration of the ion internal energy has any effect, CA
In comparison, the MI spectrum of ND OD is featured spectra of 2 were also recorded at various ionization ener-
•
ϩ
•؉
2
by two peaks at m ϭ 18 and m ϭ 20 in a 1.2:1 ratio, indicat- gies ranging from 10 to 100 V under EI conditions; in ad-
ϩ
•ϩ
ϩ
ing that ND2 as well as ND3 are produced with ND2
dition, 2 was ionized under CI conditions at higher source
•
ϩ
ϩ
slightly dominating. Further, the combined analysis of the pressures. Except for the HNO /NO ratio (see below),
results obtained for the unlabeled and the labeled ions sug- both variations, however, did not lead to significant changes
gests that contributions of oxygen-containing fragments are of fragmentation patterns, suggesting that internal energy
minor in the MI mass spectra. This is consistent with the effects can be neglected to a first approximation.
order of the IEs of the complementary fragments arising
The differences in the MI and CA spectra of the two
from [H ,N,O], i.e. IE(NH ) ϭ 11.4 eV vs. IE(OH) ϭ 13.0 precursors prove that the ion beam produced via HN /H O
3
2
3
2
[5]
•؉
eV and IE(NH ) ϭ 10.2 eV vs. IE(O) ϭ 13.6 eV.
cannot consist of 2 alone. In fact, the absence of the sig-
3
An increase of the internal energy of the molecules by nal at 16 amu in the MI spectra suggests that rather a pure
•
؉
collisional activation leads to much more abundant frag- beam of ammonia oxide (1 ) is produced. The dominant
1534
Eur. J. Inorg. Chem. 1998, 1529Ϫ1538

Existence of the Elusive Ammonia Oxide Molecule and Its Radical Cation
FULL PAPER
ϩ
,N,O]• ions generated from HN
generated [H,N,O]•ϩ ϩ H unit has an internal excess en-
Figure 5. Mass spectra of [H
O and NH
for HN /H O (a) and NH
and NH
3 3
OH, respectively, as neutral precursors; CA spectra
/
2
H
2
2
Ϫ1
ergy of at least 19.1 kcal mol . This excess energy is suf-
ϩ
ϩ
3
2
2
OH (b), NR spectra for HN
3
/H
2
O (c)
•
ϩ
ϩ
ϩ
ficient to convert a considerable fraction of [H,N,O] im-
2
OH (d), and NR /CA spectra for HN /H O (e) and
3 2
ϩ
•
ϩ
NH
2
OH (f)
mediately to NO and H . It was found that the NO /
[
•
ϩ
H,N,O] ratio varies easily with experimental conditions,
pointing to a high sensitivity of this particular ratio towards
the internal energy. Unimolecular decomposition of both
•
precursors yields, next to dominant H loss, small amounts
•
ϩ
ϩ
•ϩ
of [H,N,O] and NO , the [H,N,O] signal being higher
in each case. In contrast to that, unimolecular decompo-
؉
؉
ϩ
sition of 4 and 5 produces mainly NO and only minor
•
ϩ
[6]
amounts of HNO
cluded that at least in the low-energy regime, [H,N,O] and
NO fragments in [H ,N,O]
cation. Therefore, it can be con-
•
ϩ
ϩ
•ϩ
are not generated via
3
ϩ
[
H ,N,O] as an intermediate, but produced on a different
2
way, probably by H2 elimination; this conjecture is com-
pletely in line with the theoretical predictions. Finally, it
ϩ
should be noted that signals due to H O ions are minor
3
in CA spectra and not observed at all in MI spectra of both
ϩ
4
precursors, although fragmentation into H O and N ( S)
3
is predicted to be the thermodynamically most favorable
•
ϩ
exit channel of [H ,N,O] . Obviously, access to this frag-
3
mentation is severely hampered by the kinetic restrictions
associated with extensive isomerization and spin crossing.
After having shown that two different cationic
•
ϩ
[
H ,N,O] precursors can be distinguished, an experimen-
3
tal differentiation of the corresponding neutral counterparts
1
and 2 is attempted. One approach to neutral ammonia
oxide is based on the strategy shown in Scheme 1. Thus, a
signal corresponds to NH3• , which is in line with an precursor molecule H NϪOR is protonated with a suitable
ϩ
2
NH ϪO connectivity, whereas the dominant fragment of CI gas. If the preferred protonation site is nitrogen, and if
3
ϩ
the NH OH precursor is NH , as expected for the the group R can form a stable cation, metastable ion decay
2
2
ϩ
ϩ
NH ϪOH connectivity. In both MI and CA experiments, of H N ϪOϪR may lead to R and neutral ammonia ox-
2
3
the HN /H O couple gives rise to a much more distinct and ide (H NO). This route can be followed by performing
3
2
3
characteristic 16/17 ratio compared to NH OH. An expla- CIDI (collision induced dissociative ionization) experiments
2
nation can be derived from the cationic potential energy in which after deflection of all charged species from the
surface. The NH3• ϩ O ( P) exit channel is much lower beam, the neutrals are subsequently reionized and de-
ϩ
3
ϩ
•
Ϫ1
[12]
in energy compared to NH
1
ϩ OH (58 kcal mol vs. tected.
CIDI experiments were performed with H NO-
2
2
04 kcal mol ). Starting from 2•؉, two decomposition SiMe (9), H NOSiMe Ph, and H NOSitBuPh as precur-
Ϫ1
3 2 2 2 2
13]
paths compete. Compared to the direct bond cleavage to sors.[
Although the stabilities of the cationic leaving
ϩ
•ϩ
ϩ
ϩ
yield NH2 ϩ OH, the NH3 ϩ O exit channel is energeti- groups should increase in the order SiMe3 < SiMe Ph
<
2
ϩ
cally favored, but entropically severely disfavored due to the SitBuPh2 , meaningful CIDI spectra could only be ob-
necessity of a hydrogen migration. Especially in the MI tained for 9. This is probably a consequence of the fact
experiment, the ions can store only a relatively low amount that the effective kinetic energies of the neutral [H ,N,O]
3
of internal energy; therefore, a fraction of molecules does fragments decrease with increasing size of the precursor,
not possess enough energy to access direct bond cleavage, which lowers the reionization yields of [H ,N,O]. The MI
3
ϩ
but undergoes isomerization instead. Another indication for spectrum of H NOSiMe3 , which was generated by pro-
3
•
؉
[14]
the formation of 1 from HN /H O is the fact that the tonation of 9 with methane as CI gas,
is featured by
3
2
intensity of the NϪO cleavage relative to the base peak is losses of 15 amu (30%), 16 amu (100%), and 33 amu (50%),
much smaller for NH OH compared to HN /N O. A com- demonstrating that the heaviest neutral fragment lost is ac-
2
3
2
mon feature of the CA spectra of both precursors is that tually [H ,N,O]. The most important feature of the CIDI
3
ϩ
•ϩ
NO formation is more pronounced than [H,N,O] for- spectrum is that reionized neutral [H ,N,O] constitutes the
mation, although the NO /H /H exit channel is 18.6 kcal base peak in the spectrum. According to the MI spectrum,
no peaks with masses higher than 33 should be present in
higher than NOH /H . The theoretical results show that a pure CIDI spectrum. Small signals between 40 and 47
for [H,N,O]• production, barriers of 89.5 kcal mol for amu indicate, however, that few NR processes occur due to
3
ϩ
•
2
Ϫ1
•ϩ
Ϫ1
mol higher in energy than HNO /H and 3.8 kcal mol
2
•
ϩ
2
ϩ
Ϫ1
•
؉
Ϫ1
1
,1-elimination from 1 and of 92.3 kcal mol for 1,1- diffusion of the reionization gas through the deflector re-
elimination from 2 have to be overcome. Therefore, every gion of the mass spectrometer.
•
؉
[12]
The key issue is, however,
Eur. J. Inorg. Chem. 1998, 1529Ϫ1538
1535

M. Brönstrup, D. Schröder, I. Kretzschmar, C. A. Schalley, H. Schwarz
FULL PAPER
whether the peak at m/z 33 can be assigned unambiguously in the structurally indicative region again resembles the ob-
to reionized ammonia oxide. Mainly two concerns discard served CA pattern for the ammonia oxide cation rather
the CIDI experiment as a proof. Although theory predicts than that of ionized NH OH (Fig. 5e and 5f). Thus, at least
2
that N-protonation of 9 is favored over O-protonation by a complete isomerization 1 Ǟ 2 at the neutral stage seems
Ϫ1
1
6 kcal mol , it cannot be excluded that a fraction of mol- improbable. Further, the intense survivor ion in NR spectra
ecules is protonated at the oxygen atom, because the pro- indicates favorable Franck-Condon factors for single-elec-
tonation with methane as CI gas is strongly exothermic at tron reduction to neutral 1, being in line with the fact that
Ϫ1
Ϫ1
either side, i. e. 86.4 kcal mol and 70.4 kcal mol , respec- the geometries of neutral and cationic ammonia oxide are
[15]
ϩ
•ϩ
tively.
Thus, one might argue that the observed reionized very similar. The NH /NH3 ratio in the NR/CA spec-
2
H ,N,O] stems from the O-protonated species, which pro- trum of 2 (Figure 5f) also implies that hydroxylamine
•
؉
[
3
duces hydroxylamine concomitantly with the generation of does not isomerize during the NR experiment, in spite of
ϩ
SiMe3 . Moreover, even if exclusive initial N-protonation the much less favorable Franck-Condon overlap of cation
is assumed, one cannot rule out a possible [1,2]-H shift and neutral.
from the nitrogen to the oxygen atom prior to or concerted
with SiMe3 loss. Both objectives would be disprovable if [H ,N,O]
In conclusion, relevant points of the cationic and neutral
ϩ
•ϩ/0
surface have been calculated on a high theo-
3
it was possible to examine N-protonated Me SiNHOH, but retical level, predicting the existence of three different
3
this compound has not been synthesized yet. Due to the [H ,N,O] isomers. Information obtained from the experi-
3
concerns mentioned, for the time being the CIDI experi- ments, combined with the theoretical results, show that two
•
ϩ
ments should be regarded as an experimental hint for neu- cationic [H ,N,O] species can be distinguished in the gas
3
tral ammonia oxide rather than a proof.
phase. Furthermore, the experiments strongly suggest the
existence of neutral ammonia oxide, although a definitive
proof cannot be provided yet. This ambiguity is probably
due to (i) the fact that the differences of the NR spectra of
Scheme 1
•
؉
•؉
1
and 2
are minor suggesting a compensation of
Franck-Condon effects in conjunction with the different
fragmentation channels accessible and (ii) the failure to per-
Ϫ
ϩ
Ϫ
ϩ
form CR and/or NR experiments as far as meaningful
information about connectivity is concerned. Nonetheless,
the NR/CA experiments presented here are very much in
•
؉
favor of the generation of 1 upon ionization of HN /H O
3
2
In a different approach to generate the neutral [H ,N,O] as well as the generation of neutral ammonia oxide 1.
3
molecule, the cationic beam was neutralized with xenon,
and after removal of all ionic species from the beam, the
We are grateful to the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, and the Volkswagen-Stiftung for
neutrals were reionized with oxygen, subsequently mass-an- financial support. M. B. and I. K. acknowledge the Stiftung Sti-
[3]
alyzed and detected. The base peak of all neutralization- pendien-Fonds des Verbandes der Chemischen Industrie for Ph. D.
reionization spectra belongs to intact [H ,N,O], which dem- scholarships. We thank Dr. J. N. Harvey for helpful discussions
3
concerning the quantum chemical calculations.
onstrates the distinct stability of neutral [H ,N,O] (Figures
3
5
c and 5d). As low-mass fragments represent a super-
position of decompositions of the projectile ion, the neutral
transient, and the reionized species, structural information
of the neutrals is difficult to achieve from fragment ion
analysis. However, we note that the most intense peak in
Experimental Section
The experiments were performed with a modified VG/ZAB/HF/
AMD four-sector mass spectrometer of BEBE configuration (B
stands for magnetic and E for electric sector), which has been de-
scribed in detail elsewhere.
•
•
the lower mass region corresponds to NH2 (ND ) for
2
both precursors, in line with the theoretical predictions for
this exit channel.
The mere observation of an NR survivor ion arising from
[16]
2 3 2
NH OH and N H/H O were ionized
by a beam of electrons (100 eV) in an electron ionization source
repeller voltage ca. 30 V). NH /N O mixtures were ionized under
(
3
2
cationic ammonia oxide does, however, not prove the exist- chemical ionization conditions. The ions of interest were acceler-
ence of neutral ammonia oxide, as the structural integrity ated to 8 keV and mass-selected by means of B(1)/E(1) at a resolu-
ϩ
ϩ
might have been lost by isomerization at the neutral stage. tion of m/∆m Ն 3000; isobaric impurities, i.e. O
2
H
and N
2 5
H
•
ϩ
•ϩ
for [H
3
,N,O] or N
2
D
4
and D
2
O
2
for [D ,N,O] were either ab-
3
Therefore, the structure of reionized [H ,N,O] survivor ions
3
sent or baseline-resolved. Unimolecular fragmentations occurring
in the field-free region preceding the second magnet are referred to
as metastable ion spectra. For collisional activation experiments,
helium (80% transmission, T) was admitted to a collision cell lo-
cated between E(1) and B(2). In these MI and CA experiments, the
ionic fragments were recorded by scanning B(2). For comparison
with the NR/CA spectra (see below), MI and CA experiments were
was examined by a collisional activation experiment (Fig-
ures 5e and 5f). For H NOH and its deuterated analog, CA
2
and NR/CA spectra are practically indistinguishable, which
proves that no significant isomerization occurs at the neu-
tral stage. The corresponding NR/CA experiment with the
ammonia oxide precursor was conducted close to the detec-
tion limit of the instrument. Although the signal-to-noise also performed with B(1)/E(1)/B(2)-mass-selected ions, scanning
ratio is poor, it can be seen that the fragmentation pattern products with E(2). For NR experiments, cations were neutralized
1536
Eur. J. Inorg. Chem. 1998, 1529Ϫ1538

Existence of the Elusive Ammonia Oxide Molecule and Its Radical Cation
FULL PAPER
D. Luckhaus, J. Chem. Phys. 1997,
[
2c]
Phys. 1997, 101, 339. Ϫ
by collisions with xenon (80% T); unreacted ions were removed
from the beam. Subsequent reionization of the neutrals was
achieved by collision with oxygen (80% T), and the resulting cat-
ionic fragments were recorded by scanning B(2). For NR/CA
experiments, the survivor ions of the NR experiment were mass-
selected with B(2), collided with helium (80% T), and the produced
fragments were recorded by scanning E(2). In order to improve the
signal-to-noise ratio, 5Ϫ100 spectra were accumulated and online-
processed with the AMD-Intectra data system.
2d]
06, 8409. Ϫ [ M. N. Hughes, K. Shrimanker, Inorg. Chim.
1
Acta 1976, 18, 69.
[3] [3a]
C. A. Schalley, G. Hornung, D. Schröder, H. Schwarz, J.
Chem. Soc. Rev. 1998, 27, 91. Ϫ [ N. Goldberg, H. Schwarz,
Acc. Chem. Res. 1994, 27, 347. Ϫ [ F. Turecek, Org. Mass
Spectrom. 1992, 27, 1087.
4]
3b]
3c]
[
Two problems closely related to [H
HOOH/H OO and the HNNH/H NN tautomerism have been
investigated by mass spectrometry: For [H ,O ], see: D.
3
,N,O] concerning the
2
2
2
2
Schröder, C. A. Schalley, N. Goldberg, J. Hru sˇ a´ k, H. Schwarz,
Chem. Eur. J. 1996, 2, 1235; for [H ,N ], see: N. Goldberg, M.
2 2
ND
2
OD was obtained by repeated recrystallization of pure
OH in CH CH OD. Pure NH OH was deliberated from
OH ·HCl with NaOEt. was produced by deuterolysis
with D O. Aqueous HN was produced by dropping
concentrated sulfuric acid into a solution of NaN in water. The
concentration of HN solutions was about 1Ϫ5%. H NOSiMe
NOSiMe Ph, and H NOSitBuPh were synthesized by treating
hydroxylamine with the corresponding chlorosilanes ClSiR in di-
chloromethane.[ All chemicals were introduced in the ion source
C. Holthausen, J. Hru sˇ a´ k, W. Koch, H. Schwarz, Chem. Ber.
1993, 126, 2753.
5]
NH
NH
2
2
3
2
2
[
[17]
S. G. Lias, J. F. Liebmann, J. L. Holmes, R. D. Levin, W. G.
ND
3
Mallard, J. Phys. Chem. Ref. Data 1988, 17, 695.
3
of Mg N
2
2
3
[6]
ϩ
2
For a combined MS and theoretical study of [H ,N,O] ions,
[6a]
3
see:
D. Schröder, F. Grandinetti, J. Hru sˇ a´ k, H. Schwarz, J.
[6b]
3
,
Phys. Chem 1992, 96, 4841. Ϫ
F. Grandinetti, J. Hru sˇ a´ k, D.
[6c]
3
2
Schröder, H. Schwarz, J. Phys. Chem. 1992, 96, 2100. Ϫ
C.
H
2
2
2
2
Lifshitz, P. J. A. Ruttink, G. Schaftenaar, J. K. Terlouw, Mass
Spectrom. Rapid Commun. 1987, 1, 61.
3
18]
[7]
Our calculated dipole moments agree fairly well with the exper-
imentally determined values of 0.59 D for hydroxylamine (taken
from Selected Values of Electric Moments for Molecules in the
Gas Phase, U.S. Department of Commerce, National Bureau of
[19]
via the metal-free teflon/glass inlet system.
Ab initio quantum-mechanical calculations were performed with
IBM RS/6000 workstations and Pentium PCs using Gaussian
Standards, 1967) and of 5.0 D for Me NO (taken from: N.
3
[20]
Hacket, R. J. W. Fevre, J. Chem. Soc. 1961, 1612).
94.
For trimethylsilyl-substituted species, geometries were fully
[8]
a
The discrepancy between our calculated value for IE (2) ϭ 9.15
optimized at the HF/6-31G* level, and single-point energies were
calculated at the MP2/6-31G* level. For all other species, full ge-
ometry optimizations and frequency calculations were carried out
at the MP2/6-311G(d,p) level. The effect of using larger basis sets
and a better correlation treatment was examined by performing
single-point MP2/6-311ϩG(3df,2p) and CCSD(T)/6-311G(d,p) cal-
culations at the previously optimized geometries. These results were
eV and the literature values (9.6 eV[ and 10.0 eV ) derived
from photoelectron spectra is quite high. We ascribe it to the
absence of the 0-0 transition in the experiment, yielding an ov-
9a]
[9b]
erestimation of IE . The calculated value for IE (2) ϭ 10.72 eV
a
v
[9a] [9b]
and the experimental values (10.6 eV
much better.
and 10.56 eV ) agree
[9] [9a]
I. A. Koppel, U. M. Mölder, R. J. Pikver, Org. React. 1983,
20, 45. Ϫ [ R. E. Kutina, G. L. Goodman, J. Berkowitz, J.
Chem. Phys. 1982, 77, 1664.
10] [10a]
9b]
[21]
further used to determine extrapolated
CCSD(T)/6-
[
J. N. Harvey, M. Aschi, H. Schwarz, W. Koch, Theor. Chem.
3
11ϩG(3df,2p) energies in an approach similar to the well-known
Acc. 1998, 99, 95. Ϫ [
10b]
D. Schröder, C. Heinemann, H.
[23]
G2 method.
All data in Tables 1 and 2 include the unscaled
Schwarz, J. N. Harvey, S. Dua, S. J. Blanskby, J. H. Bowie,
MP2/6-311G** zero-point energy corrections. The relative energy
of O was determined by adding the experimental value for the P
Ǟ D excitation (1.967 eV)
Analogously, the relative energy of N was determined by adding
Chem. Eur. J., in press.
1
3
[11] [11a]
J. Hru sˇ a´ k, H. Friedrichs, H. Schwarz, H. Razafinjanahary,
11b]
H. Chermette, J. Phys. Chem. 1996, 100, 100. Ϫ [
I. Demachy, P. C. Hilberty, Chem. Phys. Lett. 1995, 247, 126.
[12] [12a]
S. Humbel,
1
[24]
3
to the calculated total energy of O.
2
M. J. Polce, S. Beranova, M. J. Nold, C. Wesdemiotis, J.
4
2
[24]
the experimental value for the S Ǟ D excitation (2.383 eV)
to
the calculated total energy of N. The minimum energy crossing
point (MECP) of H NO was located by employing a recently devel-
[12b]
Mass Spectrom. 1996, 31, 1073. Ϫ
J. L. Holmes, A. A.
4
[12c]
Mommers, Org. Mass Spectrom. 1984, 19, 460. Ϫ
P. C. Bur-
gers, J. L. Holmes, A. A. Mommers, J. E. Szulejko, J. K. Ter-
3
oped hybrid method[10]
louw, Org. Mass Spectrom. 1984, 19, 442.
which calculates energy gradients at the
[13]
Attempts to protonate F
3
CCONHOH and H
2
NOCOCF
3
lead
MP2/6-311G(d,p) level and energies of stationary points at the
CCSD(T)/6-311G(d,p) level. The geometries of both spin states
were adjusted until the energy difference between them was smaller
to loss of HF instead of the desired loss of neutral H
3
NO.
[14]
Whereas protonation with H was not feasible, use of isobutane
2
as CI gas gave the same results as with methane.
[15]
Ϫ1
The heats of reaction were determined by substracting the pro-
than 0.06 kcal mol
.
Ϫ1
ton affinity of methane (132 kcal mol ) from the calculated
proton affinity for N- and O-protonation of 9. The calculated
total energies including zero point corrections are Ϫ538.8831
ϩ
hartree for H
2 3 3 3
NOSiMe , Ϫ539.4069 hartree for H NOSiMe
ϩ
Ƞ
and Ϫ539.2033 hartree for H
3
NOHSiMe .
Dedicated to Professor Dr. Ernst Schmitz, Berlin, on the oc-
2
[
16] [16a]
C. A. Schalley, D. Schröder, H. Schwarz, Int. J. Mass Spec-
casion of his 70th birthday.
trom. Ion Processes 1996, 153, 173. Ϫ [
16b]
R. Srinivas, D. Sülzle,
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B. Kallies, R. Mitzner, J. Phys. Chem. B 1997, 101, 2959. Ϫ
1b]
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W. Koch, C. H. DePuy, H. Schwarz, J. Am. Chem. Soc. 1991,
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16c]
R. Srinivas, D. Sülzle, T. Weiske, H. Schwarz,
M. Raimondi, Int. J. Quantum Chem. 1996, 60, 393. Ϫ [1c] R.
Int J. Mass Spectrom. Ion Processes 1991, 107, 368.
R. Steudel, P. W. Schenk in Handbuch der präparativen anorga-
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1975, p. 464.
18]
Ponec, M. Krack, K. Jug, Theor. Chim. Acta 1996, 93, 165. Ϫ
[1d]
[17]
N. N. Kharabaev, V. V. Rachkovskii, A. D. Garnovkii, Zh.
Obshch. Khim. 1995, 65, 1650 (Chem. Abstr. 1996, 125,
1e]
51522x). Ϫ [ R. D. Bach, A. L. Owensby, C. Gonzales, H.
1
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[
E[CCSD(T)/6-311ϩG(3df,2p)] ഠ E[CCSD(T)/6-311G(d,p)] ϩ
[
E[MP2/6-311ϩG(3df,2p)] Ϫ E[MP2/6-311G(d,p)] ϩ ZPVE; for
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