The microwave spectrum of cyanophosphaacetylene, H2P{single bond}C{triple bond, long}C{single bond}C{triple bond, long}N

Article abstract of DOI:10.1016/j.jms.2006.10.004

The a type transitions of the microwave rotational spectra of cyanophosphaacetylene, H₂P{single bond}C{triple bond, long}C{single bond}C{triple bond, long}N, have been investigated in the frequency region between 5 and 26.5 GHz by Fourier transformation microwave (FTMW) spectroscopy. Rotational, centrifugal distortion and ¹⁴N nuclear quadrupole coupling constants have been determined. Density functional theory level ab initio calculations were performed to predict the molecular constants, and the predicted values are in good agreement with our experimentally determined results. The ¹³C and ¹⁵N isotopomer transitions were also observed. The derived r₀ structure is quite comparable to the calculated H₂P{single bond}C{triple bond, long}C{single bond}C{triple bond, long}N equilibrium geometry.

Full text of DOI:10.1016/j.jms.2006.10.004

Journal of Molecular Spectroscopy 240 (2006) 255–259
www.elsevier.com/locate/jms
Note
The microwave spectrum
of cyanophosphaacetylene, H PAC„CAC„N
2
a
b
b,
*
Lu Kang , Andrea J. Minei , Stewart E. Novick
a
Department of Natural Sciences, Union College, Barbourville, KY 40906, USA
Department of Chemistry, Wesleyan University, Middletown, CT 06459, USA
b
Received 1 August 2006; in revised form 27 September 2006
Available online 17 October 2006
Abstract
The a type transitions of the microwave rotational spectra of cyanophosphaacetylene, H
2
PAC„CAC„N, have been investigated in
the frequency region between 5 and 26.5 GHz by Fourier transformation microwave (FTMW) spectroscopy. Rotational, centrifugal dis-
14
tortion and N nuclear quadrupole coupling constants have been determined. Density functional theory level ab initio calculations were
performed to predict the molecular constants, and the predicted values are in good agreement with our experimentally determined
13
15
results. The C and N isotopomer transitions were also observed. The derived r structure is quite comparable to the calculated
0
H PAC„CAC„N equilibrium geometry.
2
ꢀ
2006 Elsevier Inc. All rights reserved.
Keywords: Microwave spectroscopy; Structure determination; Cyanophosphaacetylene; H
2
PCCCN; Fourier transform microwave spectroscopy
1
. Introduction
of the microwave cavity, ꢀ1 ms. In the present study, we
present the result of the ‘‘next’’ molecule in this series,
when considered as a target for astrophysical detection,
We recently published a pulsed-jet Fourier transform
microwave (FTMW) study of cyanophosphine, H PCN,
cyanophosphaacetylene, H PAC„CAC„N.
2
2
the first spectroscopic study, at any wavelength, of this sim-
ple five-atom phosphine derivative [1]. Almost all the small
molecules containing both phosphorous and carbon which
have been studied by high resolution microwave spectros-
copy involve multiple bonds between the carbon and the
phosphorous, usually C„P, as presented in reference [1].
The simple phosphine derivatives are as rare as hen’s teeth
in the microwave literature, perhaps because the syntheses
are difficult with immediate rearrangement in basic medi-
um, for example, N„CAPH fi H NAC„P [2]. Reactivi-
2. Experimental
The rotational spectra of H PCCCN, cyanophospha-
2
acetylene, were recorded with a pulsed-jet Fourier trans-
form microwave spectrometer based on the original
design of Balle and Flygare [3]. There is an excellent recent
review of the design parameters of FTMW spectrometers
by Grabow, et al. [4]. The details of our version of the spec-
trometer are given elsewhere [5,6]. A mixture of 0.5%
cyanoacetylene, 0.5% phosphine, and 99% neon was
expanded through a 0.8 mm diameter General Valve nozzle
followed by a 900 V pulsed discharge immediately
2
2
ty is not a concern in in situ discharge sources in FTMW
spectrometers, it is only required that the desired molecule
be stable with respect to unimolecular dissociation or rear-
rangement on the time scale of the travel time to the center
upstream of the nozzle. H PCCCN was ‘‘synthesized’’ in
2
sufficient abundance that the signal-to-noise ratio of most
of the measured microwave transitions of the normal isoto-
*
Corresponding author. Fax: +1 860 685 2211.
E-mail address: snovick@wesleyan.edu (S.E. Novick).
pomer of H PCCCN was 5 or 10 to 1 after a few thousand
2
0
022-2852/$ - see front matter ꢀ 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jms.2006.10.004

256
L. Kang et al. / Journal of Molecular Spectroscopy 240 (2006) 255–259
Table 1
Transition frequencies and assignments of the normal isotopomer of
PCCCN
H
2
0
00
0
00
J
J
F
F
Frequency
Obs ꢁ Calc
KaKc
KaKc
2
02
1
01
2
1
2
3
1
2
0
1
2
1
5287.854
5288.069
5289.129
5289.221
5291.254
ꢁ2
1
ꢁ1
0
0
3
13
03
2
12
02
3
4
2
2
3
1
7931.099
7931.475
7931.492
ꢁ4
ꢁ3
2
3
2
3
2
3
4
2
3
1
2
3
2
7932.326
7933.479
7933.692
7933.743
7935.603
ꢁ1
0
0
1
0
Fig. 1. Three hyperfine components of the
3
03 ꢁ 202 transition of
3
4
12
2
3
11
2
4
3
5
1
3
2
4
7936.056
10575.047
10575.168
10575.213
1
0
1
ꢁ1
H
2
PCCCN. This is one of the strongest of the measured transitions.
14
13
Most of our transitions required thousands of gas pulses to achieve a
signal-to-noise ratio of 10/1. This transition was obtained with 256 gas
pulses. Multiple microwave pulses were injected into the cavity during
each gas pulse. Only 3 or 4 of these microwave pulses per gas pulse
contribute significantly to this transition.
4
04
3
03
4
3
4
5
3
4
2
3
4
3
10576.833
10578.159
10578.252
10578.283
10580.070
ꢁ1
0
1
0
0
shots of the 0.6 ms duration pulsed jet. The cyanoacetylene,
HAC„CAC„N, was synthesized by the removal of
water, using P O , from commercially available propynoic
4
13
5
15
5
05
3
12
4
14
4
04
4
3
5
3
2
4
10581.175
10581.283
10581.347
0
1
0
2
5
acid amide, HAC„CACONH [7]. Shown in Fig. 1 is one
2
of the stronger transitions which was obtained after 256 gas
pulses with at least three microwave pulses during each gas
pulse.
5
4
6
4
3
5
13218.881
13218.929
13218.972
2
0
2
5
4
5
6
4
5
3
4
5
4
13221.356
13222.755
13222.805
13222.828
13224.573
ꢁ1
2
0
2
ꢁ1
3
. Results and analysis
The spectra of five isotopologues of H PCCCN were fit
using Pickett’s SPFIT suite of spectra fitting programs.[8,9]
2
5
14
6
16
6
06
4
13
5
15
5
05
5
4
6
4
3
5
13226.542
13226.581
13226.635
1
ꢁ1
0
Table 1 presents the ninety one K = 0 and K = 1 a-type
transitions and assignments for the normal isotopomer.
a
a
Tables 2–5 present the K = 0 a-type transitions of the sin-
a
15
1
3
6
5
7
5
4
6
15862.677
15862.703
15862.732
1
3
0
gly-substituted C or N isotopologues, all measured in
natural abundance. The K = 1 transitions were too weak
a
to measure for the substituted isotopologues.
A density functional B3LYP//6-311++g(d,p)(H,C,N)/
cc-pVQZ(P) ab initio optimization calculation was per-
6
5
6
7
5
6
4
5
6
5
15865.881
15867.323
15867.352
15867.370
15869.088
ꢁ2
3
ꢁ1
formed on H PCCCN [10]. This method and basis set
2
1
was chosen because it worked well for H PCN [1]. Using
ꢁ2
2
the ab initio spectroscopic constants, it was relatively easy
to locate the microwave transitions of the normal and the
minor isotopologues of cyanophosphaacetylene. All of
the measured spectroscopic constants for all five of the
6
15
7
17
7
07
5
14
6
16
6
06
6
5
7
5
4
6
15871.869
15871.894
15871.925
0
6
ꢁ3
7
6
8
6
5
7
18506.454
18506.467
18506.495
ꢁ1
studied isotopologues of H PCCCN are given in Table 6.
0
2
The A rotational constants given in Table 6 are from the
ab initio calculation. The extremely large value of
2
7
6
7
8
6
7
5
6
7
6
18510.407
18511.869
18511.893
18511.910
18513.606
ꢁ1
ꢁ2
ꢁ1
3
8
.1 MHz for DK presented in Table 6 is also the ab initio
value. Since all the transitions are a-type, D (K ) = 0, the
a
value of D made no difference to the fit.
K
ꢁ3
B and C were both determined for the normal isotopom-
er. However, for the singly-substituted isotopologues, only

L. Kang et al. / Journal of Molecular Spectroscopy 240 (2006) 255–259
257
Table 1 (continued)
Table 2
1
3
2
Transition frequencies and assignments of the isotopomer H P CCCN
0
00
0
00
F
J
J
F
Frequency
Obs ꢁ Calc
KaKc
KaKc
0
00
0
00
F
J
J
F
Frequency
Obs ꢁ Calc
KaKc
KaKc
7
16
18
08
6
15
17
07
7
6
8
6
5
7
18517.179
18517.191
18517.219
ꢁ2
2
3
03
2
02
2
3
4
1
2
3
7928.747
7928.960
7929.010
0
0
ꢁ1
0
2
ꢁ1
0
0
ꢁ1
ꢁ4
ꢁ4
2
8
8
7
7
8
7
9
7
6
8
21150.217
21150.223
21150.249
4
04
3
03
3
4
5
2
3
4
10571.850
10571.944
10571.972
8
7
8
9
7
8
6
7
8
7
21154.927
21156.410
21156.428
21156.439
21158.125
ꢁ2
ꢁ1
1
2
ꢁ1
1
ꢁ1
2
5
05
06
4
04
05
5
6
4
5
13214.918
13214.941
6
5
5
6
7
4
5
6
15857.861
15857.885
15857.906
4
ꢁ4
1
8
17
9
19
9
09
7
16
8
18
8
08
8
7
9
7
6
8
21162.479
21162.481
21162.508
7
07
8
08
9
09
6
06
07
6
7
8
5
6
7
18500.822
18500.851
18500.869
ꢁ9
ꢁ3
3
9
8
0
8
7
9
23793.974
23793.975
23793.998
ꢁ1
ꢁ2
4
7
7
8
9
6
7
8
21143.791
21143.810
21143.822
ꢁ3
1
2
1
1
1
9
8
9
0
8
9
7
8
9
8
23799.442
23800.938
23800.952
23800.962
23802.636
ꢁ1
ꢁ1
1
3
ꢁ2
ꢁ7
ꢁ1
3
8
08
09
9
0
8
9
23786.758
23786.767
1
3
1
10010
9
9
0
1
8
9
10
26429.680
26429.695
26429.704
ꢁ3
2
4
1
1
9
1
1
18
8
17
9
19
9
09
9
8
0
8
7
9
23807.758
23807.764
23807.788
0
110
010
10
9
1
9
8
10
26437.717
26437.717
26437.735
0
ꢁ2
2
Table 3
1
13
2
Transition frequencies and assignments of the isotopomer H PC CCN
0
10
9
0
1
9
10
8
9
10
9
26443.948
26445.455
26445.467
26445.475
26447.140
ꢁ1
ꢁ1
2
3
ꢁ1
0
0
1
0
00
0
00
F
J
J
F
Frequency
Obs ꢁ Calc
KaKc
KaKc
3
03
04
06
07
08
09
2
02
03
05
06
07
08
09
2
3
4
1
2
3
7922.140
7922.353
7922.405
ꢁ1
1
1
0
1
4
6
7
8
9
3
5
6
7
8
9
3
4
5
2
3
4
10563.040
10563.134
10563.163
ꢁ1
2
ꢁ1
3
ꢁ2
2
10
19
9
18
9
0
1
8
9
10
26453.039
26453.039
26453.056
1
1
5
6
7
4
5
6
15844.646
15844.673
15844.693
(
B + C) was determined. Table 6, however, presents B and
C separately. This was done by fixing (B ꢁ C) in the
spectroscopic fit. The various (B ꢁ C)’s were calculated
from the ab initio optimized structure and then this
difference was scaled by a constant, 1.01235, given by
6
7
8
5
6
7
18485.410
18485.434
18485.452
ꢁ4
ꢁ2
2
7
8
9
6
7
8
21126.171
21126.191
21126.203
ꢁ3
0
3
(
Bmeasured ꢁ Cmeasured)/(Bcalc ꢁ Ccalc
)
for the normal
isotopomer.
8
9
0
7
8
9
23766.922
23766.937
23766.945
0
1
2
3
.1. Structural analysis
Since complete isotopic substitution was not per-
1
10010
9
0
1
8
9
10
26407.657
26407.666
26407.679
ꢁ2
ꢁ3
4
1
1
formed on cyanophosphaacetylene (H, and of course,
P, not substituted); the A rotational constants were not
measured, but calculated; and (B ꢁ C)’s were extrapolat-
ed from the ab initio calculation for the minor isotopo-
logues; the determination of the structure of the
molecule involved compromises. The Kraitchman, rs,
positions [11] were determined for the three carbon
atoms and the nitrogen atom. These positions were used
to calculate the r
bond distances in Fig. 2. An r
phaacetylene was calculated using Zbigniew Kisiel’s STR-
FIT program [9]. Ten rotational constants, B and C
distances shown in the second row of
structure of cyanophos-
s
0

258
L. Kang et al. / Journal of Molecular Spectroscopy 240 (2006) 255–259
Table 4
1
3
2
Transition frequencies and assignments of the isotopomer H PCC CN
0
00
0
00
F
J
J
F
Frequency
Obs ꢁ Calc
KaKc
KaKc
3
03
04
06
07
08
09
2
02
03
05
06
07
2
3
4
1
2
3
7842.744
7842.956
7843.006
0
ꢁ1
ꢁ1
0
1
0
4
6
7
8
3
5
6
7
3
4
5
2
3
4
10457.178
10457.270
10457.302
5
6
7
4
5
6
15685.848
15685.876
15685.896
ꢁ1
ꢁ5
ꢁ2
ꢁ4
ꢁ3
2
Fig. 2. The structure of H PCCCN. The least-squares fit, r , structure; the
2
0
substitutional, r , structure; and the ab initio structures are given. See text.
s
6
7
8
5
6
7
18300.151
18300.175
18300.191
from the five studied isotopologues of cyanophosphaacet-
ylene were used to fit four bond lengths and the
PAC„C angle. These distances and the angle are given
in the first row of Fig. 2. Finally, the calculated B3LYP
7
8
9
6
7
8
20914.447
20914.465
20914.477
ꢁ2
0
2
//6-311++g(d,p)(H,C,N)/cc-pVQZ(P) [10], values of the
distances and angles are given in the third row of
Fig. 2. These three sets of structural parameters are in
good agreement with each other. The CAC bond adja-
cent to the C„N is presented in the figure as a single
bond, but its length is between that of a typical CC sin-
gle and double bond. The most striking feature of the
structure of cyanophosphaacetylene is the nonlinear
PAC„C angle of 174ꢁ. The analogous angle, PAC„N,
9
8
08
09
8
0
7
9
23528.736
23528.757
4
5
1
1
0
010
9
9
0
1
8
9
10
26143.003
26143.012
26143.020
0
ꢁ1
0
1
1
in H PCN was determined to be 175ꢁ [1]. The nitrogen
2
Table 5
Transition frequencies and assignments of the isotopomer H
1
5
analog of H PCN is cyanamide, H NCN. A detailed
2
2
2
PCCC N
structural determination of cyanamide originally deter-
mined that the NAC„N spine was linear [12]. However,
a semirigid bender analysis of inverting cyanamide has
determined that the NAC„N spine is nonlinear by
0
00
J
J
Frequency
Obs ꢁ Calc
0
1
ꢁ4
1
1
0
2
ꢁ1
KaKc
KaKc
3
4
5
6
7
8
9
1
03
04
05
06
07
08
09
2
3
4
5
6
7
8
9
02
03
04
05
06
07
08
09
7720.630
10294.170
12867.700
15441.233
18014.755
20588.271
23161.779
25735.275
ꢀ
5ꢁ [13]. To the best of our knowledge, the nitrogen
analog of cyanophosphaacetylene, aminocyanoacetylene,
H NAC„CAC„N, has not been studied in the micro-
2
wave. No spectroscopic evidence for measurable (>3 kHz)
0
010
inversion was detected in either H PCN or in H PCCCN.
2
2
Table 6
Spectroscopic constants of H
a,b
2
PCCCN
13
13
PC CCN
13
PCC CN
15
2
H PCCC N
H
2
PCCCN
H
2
P
CCCN
H
2
H
2
c
c
c
c
c
A
B
C
128236.5
128042.2
128210.7
128236.4
128212.6
d
d
d
d
d
d
d
d
1323.04891(7)
1321.51672(6)
0.0472(3)
1322.24881(5)
1320.73961(5)
1321.15552(4)
1319.63052(4)
1307.90894(4)
1306.41144(4)
1287.4969(1)
1286.0481(1)
0.0428(8)
3
h
h
h
D
D
D
J
· 10
3
f
h
h
h
h
h
h
h
h
h
h
h
h
h
JK · 10
K
4.13(3)
e
h
8.1
f
v
v
N/r
aa
ꢁ4.247(1)
—
—
6/2 kHz
bb ꢁ vcc
0.64(5)
g
91/2 kHz
22/3 kHz
21/2 kHz
20/2 kHz
a
All constants in MHz.
b
c
d
e
f
The numbers in parenthesis are the 1-standard deviation errors.
Fixed at the ab initio value.
(
B + C)/2 is fit. (B ꢁ C) is fixed to the scaled ab initio value; see text.
K
The fit is essentially insensitive to this centrifugal distortion constant. D is held at the (large) value predicted by the ab initio calculation.
The nuclear quadrupole coupling constants of N.
14
g
h
Number of transitions in the fit/standard deviation of the fit.
Fixed to the value of the corresponding constant of the normal isotopomer.

L. Kang et al. / Journal of Molecular Spectroscopy 240 (2006) 255–259
[3] T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33–45.
259
3
.2. Nuclear quadrupole coupling constant and bonding
[4] J.-U. Grabow, E.S. Palmer, M.C. McCarthy, P. Thaddeus, Rev. Sci.
Instrum. 76 (2005) 093106.
The nitrogen nuclear quadrupole coupling constant is,
[
5] A.R. Hight Walker, W. Chen, S.E. Novick, B.D. Bean, M.D.
Marshall, J. Chem. Phys. 102 (1995) 7298.
of course, a measure of the electric field gradient at the
nitrogen nucleus of the AC„N group. Nitrile vaa values
range from ꢁ2.67(5) MHz in FAC„N [14], through
[6] W. Lin, L. Kang, S.E. Novick, J. Mol. Spectrosc. 230 (2005)
93–98.
[7] F.A. Miller, D.H. Lemmon, Spectrochim. Acta 23A (1967) 1415–
ꢁ
3.32(2) MHz in H NAC„N [15], ꢁ4.224(4) MHz in
2
1
423, The propynoic acid amide was purchased from Acme Biosci-
H CAC„N [16], to ꢁ4.70783(6) MHz in HAC„N [17],
3
ence. A detailed description of the synthesis was provided by Aldo
Apponi and Jonathan Towle, private communications..
from highest to lowest electronegative binding partner to
the cyanide. This is an approximately linear relationship
between the nitrogen quadrupole coupling constant and
the electronegativity of the atom bound to the cyanide.
[8] H.M. Pickett, J. Chem. Phys. 49 (1991) 371–377.
[9] For information on this and other extremely useful spectroscopic
programs see: Z. Kisiel’s web site PROSPE - Programs for ROtational
SPEctroscopy, at http://info.ifpan.edu.pl/~kisiel/prospe.htm.
10] Gaussian Development Version, Revision E.03, M.J. Frisch, G.W.
Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M.
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Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
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Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P.
Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D.
Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S.
Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P.
Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-
Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill,
B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople,
Gaussian, Inc., Wallingford CT, 2004.
vaa of H PCCCN is ꢁ4.247(1) MHz. Since the a axis of
2
[
the molecule is only 1.5ꢁ from the CN axis, v is essentially
aa
a measure of the field gradient along the CN triple bond.
This value is squarely in the middle of the values for CN
and is typical of that of v (N) of . . .CAC„N. In contrast,
aa
v (N) in H PAC„N is ꢁ4.589(2) MHz which is similar to
aa
2
that of HCN, as the electronegativity of H and P are sim-
ilar. The AC„CA ‘‘spacer’’ between the phosphorus and
the cyanide in H PCCCN mitigates electron donation from
the phosphorus into the cyanide. However, the field gradi-
ent about nitrogen in H PCCCN is not cylindrically sym-
2
2
metrical; (vbb ꢁ v ) = 0.64(5) MHz, less than that in
cc
H PCN, 0.765(8) MHz, but not zero. The H P-group still
2
2
serves to distort the field gradient along the nitrogen in
cyanophosphaacetylene.
[
[
11] J. Kraitchman, Am. J. Phys. 21 (1953) 17–24.
12] J.K. Tyler, J. Sheridan, C.C. Costain, J. Mol. Spectrosc. 43 (1972)
Acknowledgment
2
48–261.
[13] R.D. Brown, P.D. Godfrey, B. Kleimb o¨ mer, J. Mol. Spectrosc. 114
1985) 257–273.
14] J. Sheridan, K.K. Tyler, E.E. Aynsley, R.E. Dodd, R. Little, Nature
85 (1960) 96.
15] W.G. Read, E.A. Cohen, H.M. Pickett, J. Mol. Spectrosc. 115 (1986)
16–332.
This work was supported by the Petroleum Research
Fund of the American Chemical Society.
(
[
[
1
References
3
[
[
1] L. Kang, S.E. Novick, J. Mol. Spectrosc. 225 (2004) 66–72.
2] I.S. Matveev, Zh. Strukt. Khim. 15 (1974) 145–148.
[16] S.G. Kukolich, J. Chem. Phys. 76 (1982) 97.
[17] W.L. Ebenstein, J.S. Muenter, J. Chem. Phys. 80 (1984) 3989.