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FTMW spectroscopy of the NC2O and NC3O radicals and ab initio calculations
Chemical Physics Letters 387 (2004) 116–123 www.elsevier.com/locate/cplett
FTMW spectroscopy of the NC2O and NC3O radicals and ab initio calculations Yoshihiro Sumiyoshi, Hideyuki Takada, Yasuki Endo
*
Department of Basic Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan Received 10 November 2003 Published online:
Abstract Rotational transitions of the carbon-chain radicals, NC2 O and NC3 O, have been observed for the first time by Fourie-transform microwave spectroscopy. The radicals were produced in a supersonic free jet by a pulsed discharge of acetylcyanide (CO(CH3 )(CN)) diluted in Ar for NC2 O or a gas mixture of HC3 N and O2 diluted in Ar for NC3 O. Pure rotational transitions from N ¼ 1–0 to N ¼ 3–2 for NC2 O and those from N ¼ 2–1 to N ¼ 6–5 for NC3 O, both with fine and hyperfine structures, were observed. The rotational, spin-rotation, and hyperfine coupling constants for the radicals have been precisely determined. Ab initio calculations at the RCCSD(T) level of theory considering the correlation of core electrons using the cc-pCVTZ basis sets have been performed. The present observations and the ab initio calculations revealed that both the radicals have bent structures in the ground states. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction The NC2 O and NC3 O radicals are isoelectronic with HC3 O and HC4 O, respectively, that are considered to be important intermediates in combustion processes. Furlan et al. [1] have reported that the photoflagmentation process of carbonylcyanide (CO(CN)2 ) yields NC2 O by UV light at 193 nm. Recently, Imamura and Washida have determined rate constants of the NC2 O + O2 and HC3 O + O2 reactions to explore the difference of the reactivity between the two isoelectronic radicals [2]. They have found a significant difference in the pressure dependence of the rate constants, and concluded that the HC3 O + O2 reaction proceeds by a two-body process, while a three-body process is dominant in the NC2 O + O2 reaction. Although mechanisms responsible for the difference have not been cleared yet, information on molecular and electronic structures obtained by highresolution spectroscopic studies may contribute to understand the reaction mechanism. Only the shortest member, NCO, have been studied by high-resolution spectroscopy for the NCn O carbon*
Corresponding author. Fax: +81-11-813-5454. E-mail address: [email protected] (Y. Endo).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.01.108
chain series. After the first observation of NCO by Dixon [3,4], the radical has been extensively studied mainly in the optical region to clarify its complicated vibronic structure caused by the Renner–Teller effect. High-resolution spectroscopic studies, resolving the hyperfine structure, have been also reported [5–7]. However, only the information based on theoretical studies [8,9] is available so far for longer members with n > 1. In contrast to the situation of the NCn O radicals, spectroscopic studies for the HCnþ1 O (n P 1) radicals have been made for members up to n ¼ 3 by microwave (MW) spectroscopy [10–14], where molecular structures of all the detected radicals were found to be bent in the ground states. Especially, for the HCCCO radical, Cooksy et al. [12,13] have carried out an extensive study to investigate the structure of the carbon-chain backbone by observing rotational spectra of six isotopomers in the 2 A0 ground state by millimeter-wave spectroscopy. They concluded that the radical has an acetylenic carbon-chain framework with the unpaired electron strongly localized on the carbon atom next to the ter minal O atom, H–CBC–C@O, and the backbone is bent at the CCO part, where the unpaired electron is represented as a single dot.
Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123
Quite recently, pure rotational spectra of other related carbon-chain radicals, NCn S (n ¼ 2–7), have been observed by Fourier-transform microwave (FTMW) spectroscopy [15,16], for which it takes more than ten years to observe longer members from the first observation of the shortest member, NCS, by MW spectroscopy in 1991 [17]. From the recent observations, it has been revealed that, although NC2 S has a bent structure, all other longer members (3 6 n 6 7) have linear heavyatom backbones. These linear members have alternate electronic ground states with even and odd numbers of carbon atoms: NC4 S and NC6 S have 2 P1=2 ground states, while NC3 S, NC5 S, and NC7 S have 2 P3=2 ground states like the isovalent HCnþ1 S radicals [18–23]. As a result of these studies, comparisons of the molecular and electronic structures between the two isoelectronic systems, NCn S and HCnþ1 S, have become possible, and several interesting features, similarities as isoelectronic systems or differences caused by the substitution of the CH-group by the N atom, have been revealed. Therefore, spectroscopic studies on the NCn O radicals are desirable to compare the results with those of HCnþ1 O. In the present Letter, we report the first observation of the pure rotational transitions of the NC2 O and NC3 O radicals produced in a supersonic molecular beam by FTMW spectroscopy. Precise molecular constants including the rotational, spin-rotation, and hyperfine coupling constants are obtained. High-level ab initio calculations at the RCCSD(T) level of theory considering the correlations of the core electrons using the cc-pCVTZ basis sets were carried out, and results are compared with those of the present experiment. The molecular structures in the ground state for NC2 O and NC3 O have been characterized.
117
N = 2-1 J = 2.5-1.5
F = 3.5-2.5 F = 1.5-1.5
F = 2.5-1.5
F = 1.5-0.5
18938.0
18939.0 MHz
Fig. 1. Example of the observed rotational spectra of the upper spin component, J ¼ 2:5–1:5, in the N ¼ 2–1 transition of NC2 O. The spectrum was obtained by averaging 300 pulsed discharge shots.
N = 3-2 F = 2.5-1.5 F = 3.5-2.5
J = 3.5-2.5
F = 4.5-3.5
J = 2.5-1.5 F = 3.5-2.5
2. Observation and analysis An FTMW spectrometer with a Fabry–Perot cavity combined with a pulsed-discharge nozzle (PDN) is used to observe pure rotational transitions of the radicals. Since details of the FTMW spectrometer and the PDN system have already been described in previous papers [24,25], only the experimental conditions for the present study are given here. The NC2 O radical was produced by discharging a sample gas, 0.3% of acetylcyanide (CO(CN)(CH3 )) diluted in the Ar buffer gas. A gas mixture, 0.3% of HC3 N and 0.3% of O2 diluted in Ar, was discharged to produce the NC3 O radical. Stagnation pressure was kept at about 2 atm for both the species. The pulsed discharge is induced in the PDN by applying a pulsed high voltage of 2 kV with a duration of 300 ms at a repetition rate of 5 Hz. Typical examples of the observed spectra are shown in Fig. 1 for NC2 O and in Fig. 2 for NC3 O. Although spectra of NC3 O can also be observed by discharging the acetylcyanide
14312.0
14313.0
14314.0 MHz
Fig. 2. Example of the observed rotational spectra of the upper spin component, J ¼ 3:5–2:5, in the N ¼ 3–2 transition of NC3 O. The spectrum was obtained by averaging 600 pulsed discharge shots.
sample, intensity of the spectrum is weaker by 1/3 than that obtained by the mixture gas of HC3 N and O2 . The spectral pattern of the observed rotational transitions of NC2 O is shown in Fig. 3, where rotational transitions from N ¼ 1–0 to N ¼ 3–2 are accompanied with the spin-rotation splittings of about 10 MHz together with much smaller hyperfine splittings caused by the N
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Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123
N=3-2
N=6-5
28620
28400
28410
28420
28625
28630 MHz
5-4
28430 MHz
2-1 23850
23855
23860
4-3
18930
18940
18950
18960
19075
19080
19085 MHz
1-0
3-2
14305
9460
9470
9480
14310
14315
2-1
9490
Fig. 3. Spectral pattern of the NC2 O radical.
9535
nucleus. The spectral pattern of NC3 O has much smaller spin-rotation splittings that are almost the same as those of the hyperfine splittings due to the N nucleus as shown in Fig. 4. Because of the small spin splittings, the spectral pattern is more complicated than that of NC2 O. An important result concluded from these spectral patterns is that these two radicals have bent structures in the ground states. The electron configuration of NC2 O in the ground state is . . . ð1pÞ4 ð2pÞ4 ð3pÞ1 and that of NC3 O is . . . ð1pÞ4 ð11rÞ2 ð2pÞ4 ð3pÞ3 in the limit of linearity. The ground states should be 2 Pr for NC2 O and 2 Pi for NC3 O, respectively, if they are linear, and rotational energy levels should be expressed by the HundÕs case (a) coupling scheme with the half-integer quantum number, J . However, as shown in Figs. 3 and 4, the observed rotational transitions are well expressed by the HundÕs case (b) coupling scheme with the integer quantum number, N . It is also reasonable to conclude that since the rotational temperature is cooled down to a few Kelvin under the supersonic jet condition, only the atype rotational transitions with the Ka ¼ 0 levels have been observed in the present study. Observed transition frequencies are analyzed using the ordinary 2 R Hamiltonian, 1 ð2Þ H ¼ BN 2 DN 4 þ cN S þ bF I S þ cIz Sz þ eQqT0 ðIÞ; 4
because the data are limited to those for Ka ¼ 0 and asymmetries of the two species are considered to be very small. The last three terms are the magnetic hyperfine
9540
9545
Fig. 4. Spectral pattern of the NC3 O radical.
coupling terms and the nuclear quadrupole coupling term originating from the N nucleus. Observed transition frequencies and residuals of the least-squares analyses for NC2 O are listed in Table 1, and those for NC3 O in Table 2. Standard deviations of the fits, 7 kHz for NC2 O and 8 kHz for NC3 O, are reasonable compared with the experimental accuracy. The determined effective molecular constants for the two radicals are summarized in Table 3. Ab initio calculations at the RCCSD(T);core level of theory have been carried out to obtain structural parameters in the ground states. Correlation-consistent polarized core-valence triple zata basis sets, cc-pCVTZ [26], are used in the present calculations. It has been found that using the basis set in combination with the RCCSD(T);core level of theory is quite important in the present calculations. Molecular structure of NC2 O was also calculated at the RCCSD(T)/cc-pVQZ level of theory, and it was confirmed that the calculation of the RCCSD(T);core/cc-pCVTZ level of theory provides almost the same structure as that of the RCCSD(T)/ cc-pVQZ level of theory with less computational time. The structural parameters, the rotational constants, and the electric dipole moments computed at the optimized structures are summarized in Table 4, along with the rotational constants obtained in the present experimental study for comparison. The optimized structures
Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123
119
Table 1 Observed transition frequencies of NC2 Oa N0
J0
F0
N 00
J 00
F 00
Observed
Observed ) calculated
1 1 1 1 1 1 1
1.5 1.5 1.5 1.5 0.5 0.5 0.5
0.5 1.5 1.5 2.5 1.5 1.5 0.5
0 0 0 0 0 0 0
0.5 0.5 0.5 0.5 0.5 0.5 0.5
0.5 1.5 0.5 1.5 1.5 0.5 1.5
9467.9786 9461.7613 9468.6882 9467.5293 9477.5956 9484.5252 9482.4383
).0028 .0012 .0005 .0009 .0014 .0034 ).0053
2 2 2 2 2 2 2 2 2 2 2
2.5 2.5 2.5 2.5 2.5 1.5 1.5 1.5 1.5 1.5 1.5
1.5 2.5 1.5 2.5 3.5 2.5 1.5 1.5 0.5 2.5 2.5
1 1 1 1 1 1 1 1 1 1 1
1.5 1.5 1.5 1.5 1.5 0.5 0.5 0.5 0.5 1.5 1.5
1.5 2.5 0.5 1.5 2.5 1.5 0.5 1.5 1.5 1.5 2.5
18938.5308 18933.5547 18939.2406 18939.3185 18938.8993 18947.6156 18945.1888 18950.0460 18953.7932 18963.4451 18957.6801
).0062 .0023 ).0028 ).0022 .0012 .0041 ).0013 .0065 ).0036 ).0005 .0029
3 3 3 3 3 3 3 3 3 3 3
3.5 3.5 3.5 3.5 3.5 2.5 2.5 2.5 2.5 2.5 2.5
2.5 3.5 4.5 2.5 3.5 1.5 2.5 3.5 2.5 1.5 3.5
2 2 2 2 2 2 2 2 2 2 2
2.5 2.5 2.5 2.5 2.5 1.5 1.5 1.5 1.5 1.5 2.5
1.5 2.5 3.5 2.5 3.5 0.5 1.5 2.5 2.5 1.5 3.5
28410.3699 28410.4527 28410.2478 28409.6149 28405.1073 28419.0298 28418.7875 28419.2829 28421.2358 28422.7824 28438.0593
).0067 ).0048 ).0046 .0220 ).0044 .0011 ).0073 ).0010 .0130 ).0037 ).0037
a
In MHz.
are also shown in Fig. 5. The rotational constants obtained by the present calculations agree with those determined experimentally within 1%. All the present calculations were performed using the MO L P R O 2002.3 package [27].
3. Discussion The present results, where 2 R type spectral patterns have been observed for the two radicals, indicate that the radicals have bent structures in the ground electronic states, because they should have 2 P states in the limit of linearity. Relatively large centrifugal distortion constants obtained using the effective 2 R Hamiltonian are also understood by this fact. The normalized effective centrifugal distortion constants, D=B3 , are compared with those of related carbon-chain molecules in Table 5. It is generally seen that effective centrifugal distortion constants for nonlinear molecules are larger than those of linear molecules when Ka ¼ 0 transitions are fitted to an effective 2 R Hamiltonian, because the asymmetry of the molecule gives rise to a contribution to the centrifugal distortion constant [28]. The values for NC2 O and
NC3 O in Table 5 are typical to those of bent carbonchain molecules rather than those of linear molecules. Assuming the ab initio structures shown in Fig. 5, contributions of the asymmetry to the D=B3 value is estimated to be 26.7 1015 MHz2 for NC2 O and 45.1 1015 MHz2 for NC3 O, respectively. By subtracting these contributions from the observed D=B3 values, corrected values are obtained as shown in Table 5. The corrected value of NC2 O is close to that of linear molecules. However, that of NC3 O is still large, presumably because the B–C value of the ab initio calculation (48 MHz) is underestimated. Since the D=B3 value is sensitive to the value of B–C, a small change of the rotational constants causes a significant change of the D=B3 value. If we assume the slightly larger B–C value, 57 MHz, a smaller ½D=B3 corr value, 22 1015 MHz2 , is obtained, which is close to that of NC3 S (16.7 1015 MHz2 ). The determined hyperfine constants are compared with those of the smallest member, NCO, and isovalent carbon-chain radicals, NC2 S and NC3 S in Table 6. Nitrogen quadrupole coupling constants of NC2 O and NC3 O are close to those of the NC2 S and NC3 S radicals, and are also close to the standard value of a
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Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123
Table 2 Observed transition frequencies of NC3 Oa Observed ) calculated
N0
J0
F0
N 00
J 00
F 00
2 2 2 2 2 2 2 2 2
2.5 2.5 2.5 2.5 2.5 1.5 1.5 1.5 1.5
1.5 2.5 1.5 2.5 3.5 2.5 1.5 1.5 1.5
1 1 1 1 1 1 1 1 1
1.5 1.5 1.5 1.5 1.5 0.5 0.5 0.5 1.5
1.5 2.5 0.5 1.5 2.5 1.5 0.5 0.5 1.5
9534.5024 9535.4555 9542.1360 9542.7424 9542.7973 9541.4470 9534.5216 9540.9188 9546.6481
).0012 .0036 ).0089 .0045 ).0008 ).0078 ).0057 .0119 .0041
3 3 3 3 3 3 3 3
2.5 2.5 2.5 2.5 3.5 3.5 3.5 3.5
1.5 2.5 3.5 1.5 2.5 3.5 4.5 3.5
2 2 2 2 2 2 2 2
1.5 1.5 1.5 1.5 2.5 2.5 2.5 2.5
0.5 1.5 2.5 1.5 1.5 2.5 3.5 3.5
14310.8253 14310.9886 14312.2781 14317.2446 14312.8496 14313.2904 14313.3552 14305.9362
.0009 ).0071 ).0061 .0407 ).0037 .0080 ).0017 .0000
4 4 4 4 4 4 4
3.5 3.5 3.5 4.5 4.5 4.5 4.5
2.5 3.5 4.5 3.5 4.5 5.5 4.5
3 3 3 3 3 3 3
2.5 2.5 2.5 3.5 3.5 3.5 3.5
1.5 2.5 3.5 2.5 3.5 4.5 4.5
19082.3347 19082.5616 19082.9934 19083.5236 19083.9050 19083.9527 19076.4763
.0025 ).0015 ).0075 .0005 .0149 ).0121 .0069
5 5 5 5 5 5
4.5 4.5 4.5 5.5 5.5 5.5
3.5 4.5 5.5 4.5 5.5 6.5
4 4 4 4 4 4
3.5 3.5 3.5 4.5 4.5 4.5
2.5 3.5 4.5 3.5 4.5 5.5
23853.1893 23853.4737 23853.5764 23854.0709 23854.4244 23854.5071
.0074 .0083 ).0068 ).0006 ).0051 .0064
6 6 6 6 6 6
5.5 5.5 5.5 6.5 6.5 6.5
1.5 1.5 1.5 1.5 1.5 1.5
5 5 5 5 5 5
4.5 4.5 4.5 5.5 5.5 5.5
1.5 1.5 1.5 1.5 1.5 1.5
28623.7297 28624.0000 28624.0000 28624.4487 28624.8218 28624.9166
.0050 ).0636 ).0016 ).0084 ).0142 .0138
a
In MHz.
Table 3 Molecular constants of NC2 O and NC3 Oa;b
B0 D0 c cD bF c eQq rfit (kHz) a b
Observed
NC2 O
NC3 O
4735.8730(8) 0.00392(5) )9.323(3) )0.0009(2) 4.619(3) )3.544(7) )4.482(7) 7
2385.4607(5) 0.001344(9) 0.802(5) 0.00055(9) 8.703(7) )18.11(3) )3.58(3) 8
In MHz. Values in parentheses denote one standard deviation of the fits.
terminal N atom, ca. )4 MHz, forming a typical triple bond, but are much larger than that of NCO. From the comparison, it can be concluded that the CN bond characters of NC2 O and NC3 O are similar with each
other, while they are different from that of NCO. The magnetic hyperfine constants provide information on the unpaired electron. As shown in Table 6, the bF and c constants for NC2 O and NC3 O are much smaller than those of NCO, where bF and c are obtained from the values in [7] by using the approximation, c ¼ 3ða dÞ [29]. The unpaired electron densities on the terminal N atoms have been estimated for NCO, NC2 O, NC3 O, and NC2 S by comparing gbgN bN hr3 i of the radicals with that of N atom, 138.8 MHz [30], which are also shown in Table 6. The small values for NC2 O and NC3 O indicate that they have bent structures, NBC–C @O and NBC– C@C@O, rather than linear structures, N@C@C@O and N@C@C@C@O. Furthermore, the former structures with the CBN triple bonds are consistent with the fact that the eQq values are almost the same as that of a typical terminal CBN bond. On the other hand, a larger electron density on N for NCO suggests that it has a
Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123 Table 4 Optimized geometries, dipole moments, and rotational constants of NC2 O and NC3 O at the RCCSD(T);core/cc-pCVTZ level of theory NC2 O
NC3 O
1.165 1.442 – 1.179
1.175 1.360 1.300 1.165
\NCC (°) \CCC (°) \CCO (°)
168.6 – 132.1
176.5 141.3 170.4
la (D) lb (D)
1.1 0.38
2.1 0.047
A (MHz) B (MHz) C (MHz) ~ a (MHz) B Bexp : (MHz)
178470 4789 4664 4726.5 4735.9
119116 2412 2364 2387.9 2385.5
RNC RCC RCC RCO
a
(A) (A) (A) (A)
Indicates the averaged value of B and C, ðB þ CÞ=2.
(a)
168.6
1.442 1.179 132.1
1.165 (b)
176.5
1.360
1.300
170.4
141.3 1.175
1.165
Fig. 5. The optimized molecular structures of the NC2 O (a) and NC3 O (b) radicals in the ground states at the RCCSD(T);core/cc-pCVTZ level of theory. Bond lengths are in Angstroms.
N@C@O canonical structure with the terminal CN bond with more double bond character, which agrees with the fact that the eQq value of NCO is considerably smaller than those of NC2 O and NC3 O.
121
Ab initio calculations also supports the present experimental results. Optimized geometries shown in Fig. 5 are bent for NC2 O and NC3 O. The backbone of NC2 O is bent at the CCO part with an angle of 132.1° The CO bond length and the CO bond length is 1.179 A. is close to that of CO (1.208 A) in H2 C@O. Further is close to that of more, the CN bond length, 1.165 A, the typical CBN triple bond. The optimized geometry of NC2 O (Fig. 5a) is thus described as NBC–C@O as is the case for HC3 O. The optimized geometry of NC3 O (Fig. 5b) is bent at the central carbon atom. The C3 O and C2 C3 (1.300 A) bond lengths are very (1.165 A) and CC (1.317 A) close to those of the CO (1.161 A) bond lengths in H2 C@C@O, for which the backbone consists of a typical CC and CO double bonds. These structural parameters indicate that the configuration, NBC–C@C@O, is dominant for NC3 O, and it is consistent with the small unpaired electron density on the terminal N atom. However, the C1 N bond length, 1.175 is longer than that of a typical CBN triple bond, and A, is closer to that of a the C1 C2 bond length, 1.360 A, rather than that of a typical CC double bond (1.337 A) single bond (1.534 A). Thus, although the configuration, NBC–C@C@O, is dominant for NC3 O, contribution of the configuration, N@C@C@C@O, is larger than that of NC2 O. It is generally seen that when the carbon-chain backbone becomes larger, fraction of the cumulenic configuration becomes longer and the radicals tend to be linear, as has been observed for the isovalent radicals NCn S, where NC2 S is bent and all the longer members are linear [15,16]. Barriers to linearity have also been calculated for the two species where, however, significant basis set dependence has been observed. It is generally experienced that calculations at the RCCSD(T);core level of theory provide more accurate molecular structures compared to those obtained using RCCSD(T) without considering the core electron correlation. However, it is found that
Table 5 Comparison of D=B3 valuesa Bent
D=B3 ½D=B3 corr b
3
D=B ½D=B3 corr b a
Linear
NC2 O
NC2 S
HC3 O
C3 O
36.9 10.2
29.2c
21.9d
7.0e
NC3 O
HC4 O
99.0 53.9
f
55.7
Ref. [16]. Corrected value obtained by subtracting the contribution of the B–C term of the ab initio calculations. c Ref. [16]. d Ref. [13]. e Ref. [31]. f Ref. [14]. g Ref. [15]. h Ref. [32]. b
NC3 S g
16.7
C4 O 9.8h
122
Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123
Table 6 Comparison of the hyperfine constantsa
bF c gbgN bN hr3 i eQq qe
NC2 O
NC3 O
NCOb
NC2 Sc
NC3 Sd
4.62 )3.54 5.9 )4.48 4%
8.7 )18.1 30.2 )3.58 22%
56.1 )79.4 132.3 )1.55 95%
5.50 )13.8 23.0 )4.05 10%
– – – )4.23 –
a
In MHz units except q. Obtained from the values in [7], see text for detail. c Ref. [16]. d Ref. [15]. e Calculated by comparing the value in [30]. b
calculations with RCCSD(T);core/cc-pVTZ yielded less accurate structural parameters compared to those obtained by RCCSD(T)/cc-pVTZ for the present case, presumably because the basis set is not elaborate enough to take into account the core correlation properly. This tendency is particularly remarkable in the case of NC3 O, where the barrier to linearity is too small using the ccpVTZ basis. The geometrical optimization with the RCCSD(T);core/cc-pVTZ level of theory gives a larger bond angle, \C1 C2 C3 ¼ 147.3°, and the barrier is estimated to be ca. 200 cm1 . Discrepancy of the calculated rotational constant becomes larger than that obtained by the RCCSD(T)/cc-pVTZ calculation. To avoid this problem, the basis set, cc-pCVTZ, extended to properly treat core-valence correlation effect, had to be used for the calculations at the RCCSD(T);core level of theory. It is particularly important in calculating optimized geometry, when the radical has slightly bent structure, as is the case for NC3 O. Finally, the barriers, DE ¼ Elinear Ebent , have been obtained to be 3100 cm1 for NC2 O and 320 cm1 for NC3 O and the calculated rotational constants agree well with the experimental values. While the value of 3100 cm1 for NC2 O is large enough to regard the radical as a rigid bent molecule, DE of NC3 O is very small and it is reasonable to conclude that NC3 O has a larger fraction of the cumulenic configuration. Similar basis set dependence is observed for NC2 S: two fairly different values have been reported for DE, 460 cm1 by RCCSD(T)/cc-pVTZ and 258 cm1 by RCCSD(T);core/cc-pVTZ [16]. When cc-pCVTZ is used for the RCCSD(T);core level of calculation, the calculation yields the value of 390 cm1 . ~ X ~ transition of the We have already observed the B– NC3 O radical by laser induced fluorescence (LIF) spectroscopy. Parallel transitions with Ka ¼ 1 have been observed, in addition to those with Ka ¼ 0, owing to a different supersonic jet condition from that used in FTMW spectroscopy. Only 4 cm downstream from the PDN was monitored by a laser beam for LIF spectroscopy, and the rotational temperature is higher than that of the FTMW experiment. The detailed analysis of the LIF spectra and results of a simultaneous fitting with
those of FTMW spectroscopy will be reported elsewhere.
Acknowledgements The present study was supported by a grant-in-aid by the Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grant No. 13127101). References [1] A. Furlan, H.A. Scheld, J.R. Huber, Chem. Phys. Lett. 282 (1998) 1. [2] T. Imamura, N. Washida, Int. J. Chem. Kinet. 33 (2001) 440. [3] R.N. Dixson, Proc. R. Soc. Lond., Ser. A 252 (1960) 165. [4] R.N. Dixson, Can. J. Phys. 38 (1960) 10. [5] A. Carrington, A.R. Fabris, N.J.D. Lucas, J. Chem. Phys. 49 (1968) 5545. [6] A. Carrington, A.R. Fabris, N.J.D. Lucas, Mol. Phys. 16 (1969) 195. [7] S. Saito, T. Amano, J. Mol. Spectrosc. 34 (1970) 383. [8] J.S. Francisco, R. Liu, J. Chem. Phys. 107 (1997) 3840. [9] B.S. Jursic, J. Mol. Struct. (Theochem) 460 (1999) 207. [10] Y. Endo, Hirota, J. Chem. Phys. 86 (1987) 4319. [11] Y. Ohshima, Y. Endo, J. Mol. Spectrosc. 159 (1993) 458. [12] A.L. Cooksy, J.K.G. Watson, C.A. Gottlieb, P. Thaddeus, J. Chem. Phys. 101 (1994) 178. [13] A.L. Cooksy, J.K.G. Watson, C.A. Gottlieb, P. Thaddeus, Astrophys. J. Lett. 386 (1992) L27. [14] H. Kohguchi, Y. Ohshima, Y. Endo, J. Chem. Phys. 101 (1994) 6463. [15] M.C. McCarthy, A.L. Cooksy, S. Mohamed, V.D. Gordon, P. Thaddeus, Astrophys. J. 144 (2003) 287. [16] M. Nakajima, Y. Sumiyoshi, Y. Endo, J. Chem. Phys. 118 (2003) 7803. [17] T. Amano, T. Amano, J. Chem. Phys. 95 (1991) 2275. [18] J.M. Vrtilek, C.A. Gottlieb, E.W. Gottlieb, W. Wang, P. Thaddeus, Astrophys. J. 398 (1992) L73. [19] M.C. McCarthy, J.M. Vrtilek, E.W. Gottlieb, F.-M. Tao, C.A. Gottlieb, P. Thaddeus, Astrophys. J. 431 (1994) L127. [20] Y. Hirahara, Y. Ohshima, Y. Endo, J. Chem. Phys. 101 (1994) 7342. [21] V.D. Gordon, M.C. McCarthy, A.J. Apponi, P. Thaddeus, Astrophys. J. Suppl. Ser. 138 (2002) 297. [22] M. Nakajima, Y. Sumiyoshi, Y. Endo, Chem. Phys. Lett. 351 (2002) 359.
Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123 [23] M. Nakajima, Y. Sumiyoshi, Y. Endo, Chem. Phys. Lett. 355 (2002) 116. [24] M. Iida, Y. Ohshima, Y. Endo, J. Chem. Phys. 94 (1991) 6989. [25] Y. Endo, H. Kohguchi, Y. Ohshima, Faraday Discuss. 97 (1994) 341. [26] D.E. Woon, T.H. Dunning Jr., J. Chem. Phys. 103 (1995) 4572. [27] MO L P R O 2002.3 is a package of ab initio program written by H.– J. Werner, P.J. Knowles, with contributions from J. Almof, R.D. Amos, A. Berning et al.
123
[28] W. Gordy, R.L. Cook, Microwave Molecular Spectra, Wiley, New York, 1956, p. 186. [29] C.H. Towns, A.L. Schawlow, Microwave Spectroscopy, McGrawHill, New York, 1955. [30] J.R. Morton, K.F. Preston, J. Magn. Res. 30 (1978) 577. [31] R.D. Brown, P.D. Godfrey, P.S. Elmes, M. Rodler, M.L. Tack, J. Am. Chem. Soc. 107 (1985) 4112. [32] Y. Ohshima, Y. Endo, T. Ogata, J. Chem. Phys. 102 (1995) 1493.
FTMW spectroscopy of the NC2O and NC3O radicals and ab initio calculations Yoshihiro Sumiyoshi, Hideyuki Takada, Yasuki Endo
*
Department of Basic Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan Received 10 November 2003 Published online:
Abstract Rotational transitions of the carbon-chain radicals, NC2 O and NC3 O, have been observed for the first time by Fourie-transform microwave spectroscopy. The radicals were produced in a supersonic free jet by a pulsed discharge of acetylcyanide (CO(CH3 )(CN)) diluted in Ar for NC2 O or a gas mixture of HC3 N and O2 diluted in Ar for NC3 O. Pure rotational transitions from N ¼ 1–0 to N ¼ 3–2 for NC2 O and those from N ¼ 2–1 to N ¼ 6–5 for NC3 O, both with fine and hyperfine structures, were observed. The rotational, spin-rotation, and hyperfine coupling constants for the radicals have been precisely determined. Ab initio calculations at the RCCSD(T) level of theory considering the correlation of core electrons using the cc-pCVTZ basis sets have been performed. The present observations and the ab initio calculations revealed that both the radicals have bent structures in the ground states. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction The NC2 O and NC3 O radicals are isoelectronic with HC3 O and HC4 O, respectively, that are considered to be important intermediates in combustion processes. Furlan et al. [1] have reported that the photoflagmentation process of carbonylcyanide (CO(CN)2 ) yields NC2 O by UV light at 193 nm. Recently, Imamura and Washida have determined rate constants of the NC2 O + O2 and HC3 O + O2 reactions to explore the difference of the reactivity between the two isoelectronic radicals [2]. They have found a significant difference in the pressure dependence of the rate constants, and concluded that the HC3 O + O2 reaction proceeds by a two-body process, while a three-body process is dominant in the NC2 O + O2 reaction. Although mechanisms responsible for the difference have not been cleared yet, information on molecular and electronic structures obtained by highresolution spectroscopic studies may contribute to understand the reaction mechanism. Only the shortest member, NCO, have been studied by high-resolution spectroscopy for the NCn O carbon*
Corresponding author. Fax: +81-11-813-5454. E-mail address: [email protected] (Y. Endo).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.01.108
chain series. After the first observation of NCO by Dixon [3,4], the radical has been extensively studied mainly in the optical region to clarify its complicated vibronic structure caused by the Renner–Teller effect. High-resolution spectroscopic studies, resolving the hyperfine structure, have been also reported [5–7]. However, only the information based on theoretical studies [8,9] is available so far for longer members with n > 1. In contrast to the situation of the NCn O radicals, spectroscopic studies for the HCnþ1 O (n P 1) radicals have been made for members up to n ¼ 3 by microwave (MW) spectroscopy [10–14], where molecular structures of all the detected radicals were found to be bent in the ground states. Especially, for the HCCCO radical, Cooksy et al. [12,13] have carried out an extensive study to investigate the structure of the carbon-chain backbone by observing rotational spectra of six isotopomers in the 2 A0 ground state by millimeter-wave spectroscopy. They concluded that the radical has an acetylenic carbon-chain framework with the unpaired electron strongly localized on the carbon atom next to the ter minal O atom, H–CBC–C@O, and the backbone is bent at the CCO part, where the unpaired electron is represented as a single dot.
Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123
Quite recently, pure rotational spectra of other related carbon-chain radicals, NCn S (n ¼ 2–7), have been observed by Fourier-transform microwave (FTMW) spectroscopy [15,16], for which it takes more than ten years to observe longer members from the first observation of the shortest member, NCS, by MW spectroscopy in 1991 [17]. From the recent observations, it has been revealed that, although NC2 S has a bent structure, all other longer members (3 6 n 6 7) have linear heavyatom backbones. These linear members have alternate electronic ground states with even and odd numbers of carbon atoms: NC4 S and NC6 S have 2 P1=2 ground states, while NC3 S, NC5 S, and NC7 S have 2 P3=2 ground states like the isovalent HCnþ1 S radicals [18–23]. As a result of these studies, comparisons of the molecular and electronic structures between the two isoelectronic systems, NCn S and HCnþ1 S, have become possible, and several interesting features, similarities as isoelectronic systems or differences caused by the substitution of the CH-group by the N atom, have been revealed. Therefore, spectroscopic studies on the NCn O radicals are desirable to compare the results with those of HCnþ1 O. In the present Letter, we report the first observation of the pure rotational transitions of the NC2 O and NC3 O radicals produced in a supersonic molecular beam by FTMW spectroscopy. Precise molecular constants including the rotational, spin-rotation, and hyperfine coupling constants are obtained. High-level ab initio calculations at the RCCSD(T) level of theory considering the correlations of the core electrons using the cc-pCVTZ basis sets were carried out, and results are compared with those of the present experiment. The molecular structures in the ground state for NC2 O and NC3 O have been characterized.
117
N = 2-1 J = 2.5-1.5
F = 3.5-2.5 F = 1.5-1.5
F = 2.5-1.5
F = 1.5-0.5
18938.0
18939.0 MHz
Fig. 1. Example of the observed rotational spectra of the upper spin component, J ¼ 2:5–1:5, in the N ¼ 2–1 transition of NC2 O. The spectrum was obtained by averaging 300 pulsed discharge shots.
N = 3-2 F = 2.5-1.5 F = 3.5-2.5
J = 3.5-2.5
F = 4.5-3.5
J = 2.5-1.5 F = 3.5-2.5
2. Observation and analysis An FTMW spectrometer with a Fabry–Perot cavity combined with a pulsed-discharge nozzle (PDN) is used to observe pure rotational transitions of the radicals. Since details of the FTMW spectrometer and the PDN system have already been described in previous papers [24,25], only the experimental conditions for the present study are given here. The NC2 O radical was produced by discharging a sample gas, 0.3% of acetylcyanide (CO(CN)(CH3 )) diluted in the Ar buffer gas. A gas mixture, 0.3% of HC3 N and 0.3% of O2 diluted in Ar, was discharged to produce the NC3 O radical. Stagnation pressure was kept at about 2 atm for both the species. The pulsed discharge is induced in the PDN by applying a pulsed high voltage of 2 kV with a duration of 300 ms at a repetition rate of 5 Hz. Typical examples of the observed spectra are shown in Fig. 1 for NC2 O and in Fig. 2 for NC3 O. Although spectra of NC3 O can also be observed by discharging the acetylcyanide
14312.0
14313.0
14314.0 MHz
Fig. 2. Example of the observed rotational spectra of the upper spin component, J ¼ 3:5–2:5, in the N ¼ 3–2 transition of NC3 O. The spectrum was obtained by averaging 600 pulsed discharge shots.
sample, intensity of the spectrum is weaker by 1/3 than that obtained by the mixture gas of HC3 N and O2 . The spectral pattern of the observed rotational transitions of NC2 O is shown in Fig. 3, where rotational transitions from N ¼ 1–0 to N ¼ 3–2 are accompanied with the spin-rotation splittings of about 10 MHz together with much smaller hyperfine splittings caused by the N
118
Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123
N=3-2
N=6-5
28620
28400
28410
28420
28625
28630 MHz
5-4
28430 MHz
2-1 23850
23855
23860
4-3
18930
18940
18950
18960
19075
19080
19085 MHz
1-0
3-2
14305
9460
9470
9480
14310
14315
2-1
9490
Fig. 3. Spectral pattern of the NC2 O radical.
9535
nucleus. The spectral pattern of NC3 O has much smaller spin-rotation splittings that are almost the same as those of the hyperfine splittings due to the N nucleus as shown in Fig. 4. Because of the small spin splittings, the spectral pattern is more complicated than that of NC2 O. An important result concluded from these spectral patterns is that these two radicals have bent structures in the ground states. The electron configuration of NC2 O in the ground state is . . . ð1pÞ4 ð2pÞ4 ð3pÞ1 and that of NC3 O is . . . ð1pÞ4 ð11rÞ2 ð2pÞ4 ð3pÞ3 in the limit of linearity. The ground states should be 2 Pr for NC2 O and 2 Pi for NC3 O, respectively, if they are linear, and rotational energy levels should be expressed by the HundÕs case (a) coupling scheme with the half-integer quantum number, J . However, as shown in Figs. 3 and 4, the observed rotational transitions are well expressed by the HundÕs case (b) coupling scheme with the integer quantum number, N . It is also reasonable to conclude that since the rotational temperature is cooled down to a few Kelvin under the supersonic jet condition, only the atype rotational transitions with the Ka ¼ 0 levels have been observed in the present study. Observed transition frequencies are analyzed using the ordinary 2 R Hamiltonian, 1 ð2Þ H ¼ BN 2 DN 4 þ cN S þ bF I S þ cIz Sz þ eQqT0 ðIÞ; 4
because the data are limited to those for Ka ¼ 0 and asymmetries of the two species are considered to be very small. The last three terms are the magnetic hyperfine
9540
9545
Fig. 4. Spectral pattern of the NC3 O radical.
coupling terms and the nuclear quadrupole coupling term originating from the N nucleus. Observed transition frequencies and residuals of the least-squares analyses for NC2 O are listed in Table 1, and those for NC3 O in Table 2. Standard deviations of the fits, 7 kHz for NC2 O and 8 kHz for NC3 O, are reasonable compared with the experimental accuracy. The determined effective molecular constants for the two radicals are summarized in Table 3. Ab initio calculations at the RCCSD(T);core level of theory have been carried out to obtain structural parameters in the ground states. Correlation-consistent polarized core-valence triple zata basis sets, cc-pCVTZ [26], are used in the present calculations. It has been found that using the basis set in combination with the RCCSD(T);core level of theory is quite important in the present calculations. Molecular structure of NC2 O was also calculated at the RCCSD(T)/cc-pVQZ level of theory, and it was confirmed that the calculation of the RCCSD(T);core/cc-pCVTZ level of theory provides almost the same structure as that of the RCCSD(T)/ cc-pVQZ level of theory with less computational time. The structural parameters, the rotational constants, and the electric dipole moments computed at the optimized structures are summarized in Table 4, along with the rotational constants obtained in the present experimental study for comparison. The optimized structures
Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123
119
Table 1 Observed transition frequencies of NC2 Oa N0
J0
F0
N 00
J 00
F 00
Observed
Observed ) calculated
1 1 1 1 1 1 1
1.5 1.5 1.5 1.5 0.5 0.5 0.5
0.5 1.5 1.5 2.5 1.5 1.5 0.5
0 0 0 0 0 0 0
0.5 0.5 0.5 0.5 0.5 0.5 0.5
0.5 1.5 0.5 1.5 1.5 0.5 1.5
9467.9786 9461.7613 9468.6882 9467.5293 9477.5956 9484.5252 9482.4383
).0028 .0012 .0005 .0009 .0014 .0034 ).0053
2 2 2 2 2 2 2 2 2 2 2
2.5 2.5 2.5 2.5 2.5 1.5 1.5 1.5 1.5 1.5 1.5
1.5 2.5 1.5 2.5 3.5 2.5 1.5 1.5 0.5 2.5 2.5
1 1 1 1 1 1 1 1 1 1 1
1.5 1.5 1.5 1.5 1.5 0.5 0.5 0.5 0.5 1.5 1.5
1.5 2.5 0.5 1.5 2.5 1.5 0.5 1.5 1.5 1.5 2.5
18938.5308 18933.5547 18939.2406 18939.3185 18938.8993 18947.6156 18945.1888 18950.0460 18953.7932 18963.4451 18957.6801
).0062 .0023 ).0028 ).0022 .0012 .0041 ).0013 .0065 ).0036 ).0005 .0029
3 3 3 3 3 3 3 3 3 3 3
3.5 3.5 3.5 3.5 3.5 2.5 2.5 2.5 2.5 2.5 2.5
2.5 3.5 4.5 2.5 3.5 1.5 2.5 3.5 2.5 1.5 3.5
2 2 2 2 2 2 2 2 2 2 2
2.5 2.5 2.5 2.5 2.5 1.5 1.5 1.5 1.5 1.5 2.5
1.5 2.5 3.5 2.5 3.5 0.5 1.5 2.5 2.5 1.5 3.5
28410.3699 28410.4527 28410.2478 28409.6149 28405.1073 28419.0298 28418.7875 28419.2829 28421.2358 28422.7824 28438.0593
).0067 ).0048 ).0046 .0220 ).0044 .0011 ).0073 ).0010 .0130 ).0037 ).0037
a
In MHz.
are also shown in Fig. 5. The rotational constants obtained by the present calculations agree with those determined experimentally within 1%. All the present calculations were performed using the MO L P R O 2002.3 package [27].
3. Discussion The present results, where 2 R type spectral patterns have been observed for the two radicals, indicate that the radicals have bent structures in the ground electronic states, because they should have 2 P states in the limit of linearity. Relatively large centrifugal distortion constants obtained using the effective 2 R Hamiltonian are also understood by this fact. The normalized effective centrifugal distortion constants, D=B3 , are compared with those of related carbon-chain molecules in Table 5. It is generally seen that effective centrifugal distortion constants for nonlinear molecules are larger than those of linear molecules when Ka ¼ 0 transitions are fitted to an effective 2 R Hamiltonian, because the asymmetry of the molecule gives rise to a contribution to the centrifugal distortion constant [28]. The values for NC2 O and
NC3 O in Table 5 are typical to those of bent carbonchain molecules rather than those of linear molecules. Assuming the ab initio structures shown in Fig. 5, contributions of the asymmetry to the D=B3 value is estimated to be 26.7 1015 MHz2 for NC2 O and 45.1 1015 MHz2 for NC3 O, respectively. By subtracting these contributions from the observed D=B3 values, corrected values are obtained as shown in Table 5. The corrected value of NC2 O is close to that of linear molecules. However, that of NC3 O is still large, presumably because the B–C value of the ab initio calculation (48 MHz) is underestimated. Since the D=B3 value is sensitive to the value of B–C, a small change of the rotational constants causes a significant change of the D=B3 value. If we assume the slightly larger B–C value, 57 MHz, a smaller ½D=B3 corr value, 22 1015 MHz2 , is obtained, which is close to that of NC3 S (16.7 1015 MHz2 ). The determined hyperfine constants are compared with those of the smallest member, NCO, and isovalent carbon-chain radicals, NC2 S and NC3 S in Table 6. Nitrogen quadrupole coupling constants of NC2 O and NC3 O are close to those of the NC2 S and NC3 S radicals, and are also close to the standard value of a
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Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123
Table 2 Observed transition frequencies of NC3 Oa Observed ) calculated
N0
J0
F0
N 00
J 00
F 00
2 2 2 2 2 2 2 2 2
2.5 2.5 2.5 2.5 2.5 1.5 1.5 1.5 1.5
1.5 2.5 1.5 2.5 3.5 2.5 1.5 1.5 1.5
1 1 1 1 1 1 1 1 1
1.5 1.5 1.5 1.5 1.5 0.5 0.5 0.5 1.5
1.5 2.5 0.5 1.5 2.5 1.5 0.5 0.5 1.5
9534.5024 9535.4555 9542.1360 9542.7424 9542.7973 9541.4470 9534.5216 9540.9188 9546.6481
).0012 .0036 ).0089 .0045 ).0008 ).0078 ).0057 .0119 .0041
3 3 3 3 3 3 3 3
2.5 2.5 2.5 2.5 3.5 3.5 3.5 3.5
1.5 2.5 3.5 1.5 2.5 3.5 4.5 3.5
2 2 2 2 2 2 2 2
1.5 1.5 1.5 1.5 2.5 2.5 2.5 2.5
0.5 1.5 2.5 1.5 1.5 2.5 3.5 3.5
14310.8253 14310.9886 14312.2781 14317.2446 14312.8496 14313.2904 14313.3552 14305.9362
.0009 ).0071 ).0061 .0407 ).0037 .0080 ).0017 .0000
4 4 4 4 4 4 4
3.5 3.5 3.5 4.5 4.5 4.5 4.5
2.5 3.5 4.5 3.5 4.5 5.5 4.5
3 3 3 3 3 3 3
2.5 2.5 2.5 3.5 3.5 3.5 3.5
1.5 2.5 3.5 2.5 3.5 4.5 4.5
19082.3347 19082.5616 19082.9934 19083.5236 19083.9050 19083.9527 19076.4763
.0025 ).0015 ).0075 .0005 .0149 ).0121 .0069
5 5 5 5 5 5
4.5 4.5 4.5 5.5 5.5 5.5
3.5 4.5 5.5 4.5 5.5 6.5
4 4 4 4 4 4
3.5 3.5 3.5 4.5 4.5 4.5
2.5 3.5 4.5 3.5 4.5 5.5
23853.1893 23853.4737 23853.5764 23854.0709 23854.4244 23854.5071
.0074 .0083 ).0068 ).0006 ).0051 .0064
6 6 6 6 6 6
5.5 5.5 5.5 6.5 6.5 6.5
1.5 1.5 1.5 1.5 1.5 1.5
5 5 5 5 5 5
4.5 4.5 4.5 5.5 5.5 5.5
1.5 1.5 1.5 1.5 1.5 1.5
28623.7297 28624.0000 28624.0000 28624.4487 28624.8218 28624.9166
.0050 ).0636 ).0016 ).0084 ).0142 .0138
a
In MHz.
Table 3 Molecular constants of NC2 O and NC3 Oa;b
B0 D0 c cD bF c eQq rfit (kHz) a b
Observed
NC2 O
NC3 O
4735.8730(8) 0.00392(5) )9.323(3) )0.0009(2) 4.619(3) )3.544(7) )4.482(7) 7
2385.4607(5) 0.001344(9) 0.802(5) 0.00055(9) 8.703(7) )18.11(3) )3.58(3) 8
In MHz. Values in parentheses denote one standard deviation of the fits.
terminal N atom, ca. )4 MHz, forming a typical triple bond, but are much larger than that of NCO. From the comparison, it can be concluded that the CN bond characters of NC2 O and NC3 O are similar with each
other, while they are different from that of NCO. The magnetic hyperfine constants provide information on the unpaired electron. As shown in Table 6, the bF and c constants for NC2 O and NC3 O are much smaller than those of NCO, where bF and c are obtained from the values in [7] by using the approximation, c ¼ 3ða dÞ [29]. The unpaired electron densities on the terminal N atoms have been estimated for NCO, NC2 O, NC3 O, and NC2 S by comparing gbgN bN hr3 i of the radicals with that of N atom, 138.8 MHz [30], which are also shown in Table 6. The small values for NC2 O and NC3 O indicate that they have bent structures, NBC–C @O and NBC– C@C@O, rather than linear structures, N@C@C@O and N@C@C@C@O. Furthermore, the former structures with the CBN triple bonds are consistent with the fact that the eQq values are almost the same as that of a typical terminal CBN bond. On the other hand, a larger electron density on N for NCO suggests that it has a
Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123 Table 4 Optimized geometries, dipole moments, and rotational constants of NC2 O and NC3 O at the RCCSD(T);core/cc-pCVTZ level of theory NC2 O
NC3 O
1.165 1.442 – 1.179
1.175 1.360 1.300 1.165
\NCC (°) \CCC (°) \CCO (°)
168.6 – 132.1
176.5 141.3 170.4
la (D) lb (D)
1.1 0.38
2.1 0.047
A (MHz) B (MHz) C (MHz) ~ a (MHz) B Bexp : (MHz)
178470 4789 4664 4726.5 4735.9
119116 2412 2364 2387.9 2385.5
RNC RCC RCC RCO
a
(A) (A) (A) (A)
Indicates the averaged value of B and C, ðB þ CÞ=2.
(a)
168.6
1.442 1.179 132.1
1.165 (b)
176.5
1.360
1.300
170.4
141.3 1.175
1.165
Fig. 5. The optimized molecular structures of the NC2 O (a) and NC3 O (b) radicals in the ground states at the RCCSD(T);core/cc-pCVTZ level of theory. Bond lengths are in Angstroms.
N@C@O canonical structure with the terminal CN bond with more double bond character, which agrees with the fact that the eQq value of NCO is considerably smaller than those of NC2 O and NC3 O.
121
Ab initio calculations also supports the present experimental results. Optimized geometries shown in Fig. 5 are bent for NC2 O and NC3 O. The backbone of NC2 O is bent at the CCO part with an angle of 132.1° The CO bond length and the CO bond length is 1.179 A. is close to that of CO (1.208 A) in H2 C@O. Further is close to that of more, the CN bond length, 1.165 A, the typical CBN triple bond. The optimized geometry of NC2 O (Fig. 5a) is thus described as NBC–C@O as is the case for HC3 O. The optimized geometry of NC3 O (Fig. 5b) is bent at the central carbon atom. The C3 O and C2 C3 (1.300 A) bond lengths are very (1.165 A) and CC (1.317 A) close to those of the CO (1.161 A) bond lengths in H2 C@C@O, for which the backbone consists of a typical CC and CO double bonds. These structural parameters indicate that the configuration, NBC–C@C@O, is dominant for NC3 O, and it is consistent with the small unpaired electron density on the terminal N atom. However, the C1 N bond length, 1.175 is longer than that of a typical CBN triple bond, and A, is closer to that of a the C1 C2 bond length, 1.360 A, rather than that of a typical CC double bond (1.337 A) single bond (1.534 A). Thus, although the configuration, NBC–C@C@O, is dominant for NC3 O, contribution of the configuration, N@C@C@C@O, is larger than that of NC2 O. It is generally seen that when the carbon-chain backbone becomes larger, fraction of the cumulenic configuration becomes longer and the radicals tend to be linear, as has been observed for the isovalent radicals NCn S, where NC2 S is bent and all the longer members are linear [15,16]. Barriers to linearity have also been calculated for the two species where, however, significant basis set dependence has been observed. It is generally experienced that calculations at the RCCSD(T);core level of theory provide more accurate molecular structures compared to those obtained using RCCSD(T) without considering the core electron correlation. However, it is found that
Table 5 Comparison of D=B3 valuesa Bent
D=B3 ½D=B3 corr b
3
D=B ½D=B3 corr b a
Linear
NC2 O
NC2 S
HC3 O
C3 O
36.9 10.2
29.2c
21.9d
7.0e
NC3 O
HC4 O
99.0 53.9
f
55.7
Ref. [16]. Corrected value obtained by subtracting the contribution of the B–C term of the ab initio calculations. c Ref. [16]. d Ref. [13]. e Ref. [31]. f Ref. [14]. g Ref. [15]. h Ref. [32]. b
NC3 S g
16.7
C4 O 9.8h
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Y. Sumiyoshi et al. / Chemical Physics Letters 387 (2004) 116–123
Table 6 Comparison of the hyperfine constantsa
bF c gbgN bN hr3 i eQq qe
NC2 O
NC3 O
NCOb
NC2 Sc
NC3 Sd
4.62 )3.54 5.9 )4.48 4%
8.7 )18.1 30.2 )3.58 22%
56.1 )79.4 132.3 )1.55 95%
5.50 )13.8 23.0 )4.05 10%
– – – )4.23 –
a
In MHz units except q. Obtained from the values in [7], see text for detail. c Ref. [16]. d Ref. [15]. e Calculated by comparing the value in [30]. b
calculations with RCCSD(T);core/cc-pVTZ yielded less accurate structural parameters compared to those obtained by RCCSD(T)/cc-pVTZ for the present case, presumably because the basis set is not elaborate enough to take into account the core correlation properly. This tendency is particularly remarkable in the case of NC3 O, where the barrier to linearity is too small using the ccpVTZ basis. The geometrical optimization with the RCCSD(T);core/cc-pVTZ level of theory gives a larger bond angle, \C1 C2 C3 ¼ 147.3°, and the barrier is estimated to be ca. 200 cm1 . Discrepancy of the calculated rotational constant becomes larger than that obtained by the RCCSD(T)/cc-pVTZ calculation. To avoid this problem, the basis set, cc-pCVTZ, extended to properly treat core-valence correlation effect, had to be used for the calculations at the RCCSD(T);core level of theory. It is particularly important in calculating optimized geometry, when the radical has slightly bent structure, as is the case for NC3 O. Finally, the barriers, DE ¼ Elinear Ebent , have been obtained to be 3100 cm1 for NC2 O and 320 cm1 for NC3 O and the calculated rotational constants agree well with the experimental values. While the value of 3100 cm1 for NC2 O is large enough to regard the radical as a rigid bent molecule, DE of NC3 O is very small and it is reasonable to conclude that NC3 O has a larger fraction of the cumulenic configuration. Similar basis set dependence is observed for NC2 S: two fairly different values have been reported for DE, 460 cm1 by RCCSD(T)/cc-pVTZ and 258 cm1 by RCCSD(T);core/cc-pVTZ [16]. When cc-pCVTZ is used for the RCCSD(T);core level of calculation, the calculation yields the value of 390 cm1 . ~ X ~ transition of the We have already observed the B– NC3 O radical by laser induced fluorescence (LIF) spectroscopy. Parallel transitions with Ka ¼ 1 have been observed, in addition to those with Ka ¼ 0, owing to a different supersonic jet condition from that used in FTMW spectroscopy. Only 4 cm downstream from the PDN was monitored by a laser beam for LIF spectroscopy, and the rotational temperature is higher than that of the FTMW experiment. The detailed analysis of the LIF spectra and results of a simultaneous fitting with
those of FTMW spectroscopy will be reported elsewhere.
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