A possible role for triplet H2CN+ isomers in the formation of HCN and HNC in interstellar clouds

TL;DR: The structure and energies of the lowest triplet states of four isomers of H2CN+ have been determined by self-consistent field and configuration interaction calculations as discussed by the authors, and it is suggested that the triplet H2NC+ isomer may play a role in determining the relative yields of HCN and HNC from the reaction of C+ and NH3.

Abstract: The structures and energies of the lowest triplet states of four isomers of H2CN+ have been determined by self‐consistent field and configuration interaction calculations. When both hydrogen atoms are attached to the nitrogen atom, H2NC+, the molecule has its lowest triplet state energy, which is 97.2 kcal mol−1 above the energy of the linear singlet ground state. The structure has C2v symmetry, with an HCH bond angle of 116.8°, and bond lengths of 1.009 A (H–N) and 1.268 A (N–C). Other isomers investigated include the H2CN+ isomer at 104.7, the cis‐HCNH+ isomer at 105.3, and the trans‐HCNH+ isomer at 113.6 kcal mol−1. The H2CN+ isomer has an unusual ’’carbonium nitrene’’ structure, with a C–N bond length of 1.398 A. It is suggested that the triplet H2NC+ isomer may play a role in determining the relative yields of HCN and HNC from the reaction of C+ and NH3. Specifically, a triplet path is postulated in which C+ and NH3 yield the triplet H2NC+ isomer, which then yields the singlet H2NC+ isomer by phospho...

Summary

THEORETICAL APPROACH

• The authors first investigations were of the restricted self~consistent-field the standard contracted double zeta (DZ) Gaussian basis set of and Dunning, 23 It is C (9s5j;/4s2p), N (9s5j;/ H Variation of bond and bond angles to find the geometry of the lowest triplet the for open~shell calculations.
• To determine the orbital occupancy having the lowest energy for each the initial cal.cu~.
• \Vhere it is n.ot nee~ symmetry pattern fo:r the lations were essary to and orbitals.
• Once the optimum geometry was determined, it was fixed and additional computations were made at three higher levels of theory.
• The orbital exponents of these polarization functions were 1. 0 for the j; orbitals on hydrogen, and 0, 75 for the d orbitals on carbon and nitrogEm, Finally, the double zeta SCF function was extended by configuration interaction + P + CI) including all.

Structures

• The bond lengths and bond angles of the triplet states are shown in Fig. 3 .
• All are stable to distortion from a planar geometry.
• The Lewis structures corresponding to the above interpretation are also shown in Fig. 3 .

Total energies

• The energy of the lowest triplet state of each isomer (relative to the energy of the linear singlet ground state) is listed in Table III for each of the four levels of theory.
• At each level of theory the CNH2.
• Isomer has the lowest energy and the trans -HCNH+ isomer has the highest energy.
• P level it has practically the same energy as the CNH2 isomer.
• The fact that the CNH2 isomer is lower in energy than either of the HCNH+ structures stands in contrast to the relative energies of the singlet where the linear HCNH+ lies well below both the CNH2 and the H 2 CW isomers.

H

• As found for the triplet states of the isoelectronic acetylene molecule, the lowest cis structure has a H H lower energy than the lowest trans structure.
• The energies of both the cis and trans triplets are substantially higher (compared to the linear singlet ground state) for HCNH than for HCCH.
• For both the cis and tJans isomers the atoms bonded to nitrogen have substantial positive net atomic charges; repulsion between these charges would be ex~ pected to lead to relatively large CNH bond angles.
• On the other hand, the atoms bonded to carbon have charges of opposite sign (the charge on one atom, i.e., nitrogen, being very close to zero), and therefore the HCN bond angles would be expected to be somewhat smaller.

Mulliken populations

• The H 2 Nc singlet isomer can react directly with an electron or a base [Reactions (2) and (5)~( 8)], or else it can first isomerize to the linear structure.
• This point will be discussed further in the next section.
• If this energy has not been entirely removed by colli, sional deactivation, then Reaction (8) may occur.
• Other reactions to be considered are the direct formation of HCN'' (or HNC') followed electron capture from an atom or molecule having a lower ionization ,Jv,,v, . EQUATION Reaction (11) has a much smaller rate constant than Reaction (1).

Relative abundances of HCN and HNC

• 32 In some clouds the HNC abundance is significantly greater than that of HCN.
• 38 To explain the relatively high abundance of HNC, it has been suggested that when the linear singlet ground state undergoes Reaction (2), the CH and NH bonds rupture with about the same probability, leading to the pro~, duction of approximately equal amounts of HCN and HNC.
• This could be done precisely if one determined the vibrational frequencies of triplet H 2 NC', or assumed that they are similar to those of another molecule (formaldehyde, for example).
• Let us now consider the energy changes in these reactions.
• On the other hand, a large majority of molecules following the singlet path will have sufficient energy to isomerize to the linear singlet ground state, and therefore the singlet path will yield both HCN and HNC in approximately equal amounts.

Deuterium isotope effect

• Another remarkable feature of the chemistry of interstellar clouds is the very large deuterium enrichment of HCN and HNC.
• Rather than a mechanism for deuterium enrichment of either HNC or HCN, what seems to be needed is a mechanism for the deuterium depletion of the HCN isomer, which appears to have a deuterium content much lower than that of HNC.
• The most obvious candidate is the following.
• Because of its relative instability, all observed HNC must be relatively new.
• When there are two or more channels for the products of two reacting species, then there exists the possibility of relatively large isotopic variations in the relative rates of the different channels.

Figures

FIG. 2. Orbital energies of the lowest triplet states of cisand trans-acetylene and dihydrogen cyanide cation from a DZ +Pbasis.

TABLE II. Orbital energies of the lowest singlet and triplet states of the H2CN+ molecule, obtained with the DZ basis. (Geometries of the singlet states are listed in Ref. 14.) The top half of the table lists the first four orbitals for each isomer; the bottom half lists the other occupied orbitals. For the states, the last two orbitals are

FIG. 3. Bond 1 engths and bond angles of the lowest triplet states for four isomers of

FIG. 5. Helative energies (in kcalmol-1) of the system and related molecules. Data for i.n its linear singlet ground state [denoted HCNH'(S) l arc from Hef. 88 (see also Hc£s. 29-31). At298K, of is225.0kcaimol .. 1; correction to 0 K assuming H238 K -·HoK of is the same as that of acetylene decreases this value to 224. '1 kcal. mol-1. Data for the singlet isomer are from Ref. H; for the triplet states (T) of from this work; for I-INC from Hef. 32; ionization potentials of C and N from ioni7.ation potentials of J:-1 2, and CH3 from data from JANAF tables. The various thermochemical data ·are summarized in the Appendix.

FIG. 1. Orbital energies of the lowest singlet and triplet states of CNH2 and H2cN• isomers from aDZ basis.

TABLE I. Electronic configurations and excitations from the ground state for the lowest triplet states of four isomers of H2CN+.

Full text

Lawrence Berkeley National Laboratory
Lawrence Berkeley National Laboratory
Title
A POSSIBLE ROLE FOR TRIPLET H2CN+ ISOMERS IN THE FORMATION OF HCN AND HNC IN
INTERSTELLAR CLOUDS
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https://escholarship.org/uc/item/99q1h2ct
Author
Allen, Thomas L.
Publication Date
1980-02-01
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University of California

Published
in
the
Journal
of
Chemical
Physics,
Vol.
73,
No.
7,
1
October
1980,
pp.
3255-3263
A POSSIBLE
ROLE
FOR
TRIPLET H
2
CN+
ISOMERS
IN
THE
FORMATION
OF
HCN
AND
HNC
IN
INTERSTELLAR
CLOUDS
Thomas
L.
Allen,
John
D.
Goddard,
and
Henry
F.
Schae
,
III
February
1980
TWO-WEEK
LOAN
COPY
is
a Library Circulating
Copy
which
may
be
borrowed
two
weeks.
a
personal
retention
copyy
Divisiony
Ext.
6782.
Prepared for the U.S. Department
of
Energy under Contract W-7405-ENG-48
LBL-11923
C.:l--
Reprint
-
-

DISCLAIMER
This document was prepared
as
an account
of
work sponsored by the United States
Government. While this document
is believed to contain
conect
information, neither the
United States Government
nor any agency thereof, nor the Regents
of
the University
of
California, nor any
of
their employees, makes any waJTanty, express or implied, or
assumes any legal responsibility for the accuracy, completeness, or usefulness
of
any
information, apparatus, product, or
process disclosed, or represents that its use would not
infringe privately owned rights. Reference herein to any
specific commercial product,
process,
or
service by its trade name, trademark, manufacturer, or otherwise, does not
necessarily constitute or imply its endorsement, recommendation, or
favoring by the
United States Government or any agency thereof, or the Regents
of
the University
of
California. The views and opinions
of
authors expressed herein do not necessarily state or
reflect those
of
the United States Government or any agency thereof or the Regents
of
the
University
of
California.

Thomas
L.
Allen
Department
of
Chemistry, University
of
California, Davis, California 95616
John
D.
Goddard
and
Henry
F.
Schaefer Ill
Department ofChemislly, University
of
California, Berkeley, California
94720
and
Lawrence Berkeley Laboratory, Berkeley, California
94720
(Received
26
February
1980;
accepted
20
June
1980)
The
structures
and
energies
of
the
lowest
triplet
states
of
four
isomers
of
H
2
CN
1
have
been
determined
by
self-consistent
field
and
configuration interaction calculations.
When
both hydrogen
atoms
are
attached
to
the nitrogen
atom,
H,NC',
the
molecule
has
its
lowest triplet state energy,
which
is
97.2
kcalmol-'
above
the
energy
of
the linear singlet
ground
state.
The
structure
has
C,,. symmetry,
with
an
HCH
bond
angle
of
116.8",
and
bond
lengths
of
1.009 A
(H--N)
and
1.268 A (N-C). Other
isomers
investigated
include
the
H,cN+ isomer
at
104.7,
the
cis-HCNH+
isomer
at
105.3, and
the
trans-HCNH+
isomer
at
113.6
kcal
mol-'.
The
H,CN+ isomer
has
an
unusual
"carbonium nitrene" structure,
with
a
C--N
bond
length
of
1.398
A.
It
is
suggested
that
the
triplet H
2
NC
1
isomer
may
play
a
role
in
determining
the
relative
yields
of
HCN
and
HNC
from
the
reaction
of
c+
and
NH
3
.
Specifically,
a triplet path is postulated
in
which
c+
and
NH
3
yield
the
triplet H,Nc+
isomer,
which
then
yields
the singlet H,Nc+ isomer
hy
phosphorescent emission.
Because
this
emission
removes
a
large
amount
of
energy, the singlet H
2
NC+
isomer
may
have
insufficient
energy
to
isomerize
to
the
linear singlet
ground
state. Subsequent dissociative recombination
would
yield
the
HNC
isomer
exclusively.
INTRODUCTION
While
the
dihydrogen
cyanide
cation
H
2
CN+
has
not
been
detected
in
interstellar
space
thus
far,
it
is
com-
monly
postulated
as
the
immediate
precursor
to
hydro-
gen
cyanide
HCN
and
hydrogen
isocyanide
HNC,
both
of
which
have
been
found
in
significant
quantities
in
inter-
stellar
clouds.
1
-
12
The
structures
and
energies
of
the
lowest
singlet
states
of
three
isomers
of H
2
CW
have
previously
been
calculated
by
the
techniques
of
molecu-
lar
quantum
mechanics,
3
•
13
•
14
but
nothing
is
known of
the
triplet
states.
Due
to
their
possible
importance
in
the
solution
of
various
problems
associated
with
the
forma-
tion
of HCN and HNC,
as
well
as
their
intrinsic
impor-
tance,
particularly
in
comparison
to
the
triplet
states
of
the
isoelectronic
molecule
acetylene,
15
-
20
we
have
cal-
culated
the
structures
and
energies
of
the
lowest
triplet
states
of
four
isomers
of
THEORETICAL
APPROACH
Our
first
investigations
were
of
the
restricted
self~-
consistent-field
the
standard
contracted
double
zeta
(DZ)
Gaussian
basis
set
of
and
Dunning,
23
It
is
C
(9s5j;/4s2p),
N
(9s5j;/
H
Variation
of
bond
and
bond
angles
to
find
the
geometry
of
the
lowest
triplet
the
for
open~shell
calculations.
25
To
determine
the
orbital
occupancy
hav-
ing
the
lowest
energy
for
each
the
initial
cal.cu~·
\Vhere
it
is
n.ot
nee~
symmetry
pattern
fo:r
the
lations
were
essary
to
and
orbitals.
the
symmetries
of
the
various
sequent
calculations
were
made
in
or
symmetry,
At
the
geometries,
forces
in
Cartesian
coor~·
dinates
were
all
less
than
0. 3
Once
the
optimum
geometry
was
determined,
it
was
fixed
and
additional
computations
were
made
at
three
higher
levels
of
theory.
First,
the
SCF
function
was
extended
by
configuration
interaction
(DZ
+CI),
includ-
ing
all
single
and
double
excitations,
and
with
the
two
occupied
orbitals
of
lowest
energy
(core
MO)
and
the
two
virtual
orbitals
of
highest
energy
(core
complements)
frozen,
Next, a
more
comprehensive
SCF
calculation
(DZ +
was
carried
out
with
the
addition
of
polariza-
tion
functions
(a
set
of
three
jJ
-type
functions
for
each
hydrogen
atom
and
a
set
of
five
d-type
functions
each
for
carbon
and
nitrogen),
This
basis
set
is
designated
C
(9s5p1d/4s2jJld),
N
(9s5fJld/4s2jJ1d),
H
(4sljJ/2s1jJ),
The
orbital
exponents
of
these
polarization
functions
were
1.
0
for
the
j;
orbitals
on
hydrogen,
and
0,
75
for
the
d
orbitals
on
carbon
and
nitrogEm,
Finally,
the
double
zeta
SCF
function
was
extended
by
con-
figuration
interaction
+ P + CI)
including
all.
single
and
double
excitations.
Again,
the
two
occupied
orbitals
of
lowest
energy
and
the
two
virtual
orbitals
of
highest
energy
were
frozen.
total.
number
of
In
these
largest
calculations
the
for
the
two
isomers
with
and
4488
and
8878
for
the
two
isomers
with
symmetry.
All
CI
cal-
culations
\VEn·e
·with
the
group
approach
ei.tation
than
the
other
three
isomers.
different
excitations
the
and
cis-
rnay
different
ex-
However,
the
isomers
J.
Chem.
Phvs.
7317\
1 Or.t. HlRO
00?1-!:JROR/R0/1
Q<\?<;t;.OQ<i;01
00
(C)
arru:u·if'~n
lnditlltP
nf
Ph\Jt;;.ir.;:;

3256
Allen, Goddard, and Schaefer: HCN and
HNC in interstellar clouds
TABLE
I.
Electronic
configurations
and
excitations
from
the
ground
state
for
the
lowest
triplet
states
of
four
isomers
of H
2
CN+.
Isomer
CNHz
H
2
CN+
cis-HCNH+
trans-·HCNH+
Symmetry
Electronic
configuration
lal2af3a{4allb55a
1
1b[2b
2
Jaj2al3al4a\Ibl5aJ1b
1
2b
2
Excitation
5a
1
~
2b
2
1b
1
~
2b
2
Ga'-?a'
Ga
'-?a'
Excitation
in
\ia
'~?a'
la"-
.....
7a'
6a'-7a
1
6a'--7a'
===============·~~~~----~--
are
consistent
with
the
different
patterns
of
orbital
en-
.
ergies
in
the
corresponding
lowest
singlet
states
{see
Fig.
1).
For
singlet
CNH;
the
highest
occupied
orbital
is
5ai>
and
the
excitation
in
the
triplet
is
from
5a
1
to
2b
2
•
For
singlet
H
2
CW
the
highest
occupied
orbital
is
lb
1
,
and
the
excitation
in
the
triplet
state
is
from
1b
1
to
2b
2
•
(For
each
isomer
the
2b
2
orbital
is
the
lowest
virtual
orbital
of
the
singlet
state.)
Table
II
lists
the
orbital
energies
of
the
various
sing-
let
and
triplet
states,
obtained
with
the
DZ
basis.
As
found
for
the
triplet
states
of
acetylene,
20
the
singly
oc-
cupied
orbitals
of
both
the
cis
and
trans
isomers
are
well
separated
in
energy.
Figure
2
shows
a
compari-
son
of
the
cis
and
trans
orbital
energies
of
the
two
mole-
cules.
Structures
The
bond
lengths
and
bond
angles
of
the
triplet
states
are
shown
in
Fig.
3. All
are
stable
to
distortion
from
a
planar
geometry.
Also,
the
CNH2 and H
2
CW
isomers
are
stable
to
in-plane
distortions
from
C
2
v
symmetry.
Three
isomers
have
C-N
bond
lengths
in
the
range
1.
27-1.29
A,
only
slightly
longer
than
the
lengths
of
1.
23-1.
26.A
previously
found
for
the
singlet
CNH;
and
E
·0.2
2b2
-
--
-2b2
-0.4
·0.6
-0.8
-W--sa
1
+2b2
2b2
-+t-1b'j
-+-t-1b1
=rf(1b1
-+t-1b1
+t-sa
1
Sa1
·1.0
-;{--Sa1
-+t-1b2
--l+1b2
+t-1b2
--l+1b2
_,_,
r
+t-4a1
+t4a1
+t-4a
1
-4-i-
4a1
SINGLET
TRIPLET SINGLET
TRIPLET
CNH2+ISOMER H
2
CN+ISOMER
FIG.
1.
Orbital
energies
of
the
lowest
singlet
and
triplet
states
of
CNH2
and
H
2
cN•
isomers
from
aDZ
basis.
H
2
CN''
isomers.
14
We
interpret
these
bond
lengths
as
indicating
essentially
double-bond
character
in
all
cases.
(The
sum
of
the
double-bond
radii
27
<a)
is
1.
29
A.)
For
the
H
2
CW
triplet
the
C
-N
bond
length
is
substantially
longer
(1. 398
A),
which we
interpret
as
indicating
large-
ly
single-bond
character.
(The
sum
of
the
single-bond
radii,
corrected
for
electronegativity
difference,
27
<a)
is
1. 47
A.
On
using
the
Pauling
relation
between
bond
length
and bond
order,
27
<b)
modified
to
fit
a
single-bond
length
of 1. 47 A
and
a
double-bond
length
of
1.
29A, a
bond
order
of
1.
32
is
obtained
for
the
C
-N
bond of
the
H
2
CN+
triplet.)
The
reason
why
excitation
from
the
singlet
to
the
triplet
weakens
the
C --N bond in
the
H
2
CW
isomer
is
that
excitation
occurs
from
the bonding
lb
1
orbital.
(The
singly
occupied
lb
1
orbital
retains
some
of
its
bonding
character,
and
this
accounts
for
the
fact
that
the
C
-N
bond
order
in
the
triplet
is
somewhat
larger
than
one.)
By
contrast,
excitation
in
the
CNH2
isomer
occurs
from
the
nonbonding
5a
1
orbital
1
leaving
the
C-N
double
bond
intact.
The
Lewis
structures
corresponding
to the above
in-
terpretation
are
also
shown
in
Fig.
3.
If
these
struc-
tures
correctly
represent
the
main
features
of
the
elec-
.:t
I
I
¥1b'j
*4ag
·0.4~
3b2
1au
-0.6
~
---l-4a
1
~3bu
4j-3a1
-+i-----3ag
-c-i-7a'
-+t-2b2
---l-7a'
-4-t-2bu
-0.8
--G-1a"
-4-1a"
·1.0
-e-l-Sa'
df<6a'
·1.J
-I-t-sa'
Sa'
+t-4a'
-+t-4a'
cis·
cis-
trans-
trans-
HCNH+
HCCH
HCNH+
HCCH
FIG.
2.
Orbital
energies
of
the
lowest
triplet
states
of
cis-
and
trans-acetylene
and
dihydrogen
cyanide
cation
from
a DZ
+Pbasis.
J. Chern. Phys., Vol. 73,
No.7,
1 October 1980