4614
J. Phys. Chem. 1993,97, 4614-4615
Proton Affinity of CNO Robert G. A. R. Maclagan Department of Chemistry, University of Canterbury, Christchurch, New Zealand Received: October 24, 1992; In Final Form: February 10, 1993
The proton affinities at 0 K of CNO and NCO have been calculated using the G2 procedure. In order to determine the reliability of this procedure with this type of molecule, the proton affinities of CO, CN, and N O have also been calculated. The lowest energy isomer for HNCO+ was found to be the linear structure.
Introduction
1212
C-N-0
The isomers of HCNO and its ion HCNO+ are of interest as the simplest species containing all four of the elements C, H, N, and 0. HCNO has been observd i n interstellarclouds.' HNCO+ was included in a dynamical model of molecular clouds by Neyad et ala2Mass spectrometric studies of [C,H,N,O]+ isomers have been reported by Hop et al.3 and Bogan and Hand.4 The proton affinities of CNO and NCO are of interest to experimentalists. The structures and electron affinities of CNO and NCO were studied by Koch and Frenki~~g.~ An extensivestudyof the isomers of HCNO was made by Poppinger et ale6 Bogan and Hand reported INDO calculations on the isomers of HCNO+. They found a cyclic structure to be lower in energy than the linear isomer by 0.380 au. We report here the results of calculations of the proton affinity of CNO and NCO at 0 K using the G2 procedure developed by Pople's groupa7In addition, since the original paper detailing the G2 procedure did not include calculations of the proton affinities of molecules like CN, NO, or CO, we report calculations of the proton affinities of these, which allows us a better estimate of the reliability of the calculations for CNO and NCO.
Computational Details All calculations were done using the Gaussian 90 program.8 The calculations followed the prescription detailed in the original description of the G19 and Gz7 procedures.
Results and Discussion TheGl energieswith thevariouscontributions to theG1 energy are given in Table I. The G2 correction and the G2 energies are given in Table 11. Harmonic vibrational frequencies calculated at the HF/6-31G* level of theory are given in Table 111. The MP2/6-31G* optimized geometries are shown in Figure 1. Protonation lengthens the C-N bond in both CNO and NCO by about 0.07A. TheC-ObondinNCOisshortened byprotonation. For neither CNO nor NCO is protonation favored at the 0 atom.
1.228
1.253
1.186
N-C-O
H 1.037\ 117.3' 1.084 H-C -
1.144
1 N C -.
N -1.233 0
1.122
1.32SJ 170.4O
0
H
175.8O
H \1.094
c)
150.80
/
\1.333 /52.4*+
0-
1.507
N
Figure 1. MP2/6-31G1 optimized geometries of CNO and HCNO+ isomers and related species (except for CNOH+ which is HF/6-31G* optimized).
The difference is 44.1 kJ mol-' for NCO using the G2 procedure. The other protonated form of CNO, CNOH+, has an E(HF/ 6-31G*) of -167.244 49 hartrees compared with -167.306 09 hartrees for HCNO+, a difference of 198 kJ mol-I. The cyclic HCNO+ isomer suggested by Bogan and Hand4to be more stable than HNCO+ has an E(HF/6-31G1) of -167.278 63 hartrees, 266 kJ mol-' higher than HNCO+. This differenceis only reduced to 221 kJ mol-I with MP2/6-31G* calculations. Whereas HCNO+ is linear, the linear HNCO+ structure is a transition state, involving the CNH bend, lying 27.3 kJ mol-] above the bent structure. The N-C bond length is much shorter in the transition state, being 1.239 A. The calculated proton affinities at 0 K are given in Table IV. For CNO the A,??(+) and A,??(QCI) corrections to the calculated proton affinity almost cancel out each other. The correctionto the MP4/6-3 11G** + ZPVEvalue is only 5.2 kJ mol-'. For NCO the A,??(QCI) contribution is
TABLE I: G1 Energies and Correctionsa MP4/6-311GZ* W+) -167.549 21 -5.73 -167.826 84 -3.29 -167.652 87 -6.51 -167.901 10 -3.18 -167.864 80 -3.61 -92.494 82 -2.96 -92.705 16 -0.76 -92.753 66 -0.86 co -113.177 22 -3.72 COH+ -1 13.270 18 -1.43 NO -129.642 80 -6.08 NOH+ 129.831 36 -2.58 Energies in hartrees, corrections in millihartrees. molecule CNO HCNO+ NCO HNCO+ NCOH+ CN HCN+ HNC+
W2df) -88.05 -85.52 -88.72 -82.3 1 -83.98 -45.63 -40.80 41.63 -54.20 -50.92 -65.81 -59.72
0022-36S4/93/2097-4614$04.00/0
WQCI) -3.53 -6.55 -0.18 -0.53 -2.76 -19.23 -13.42 -1 1.33 +5.06 +1.34 +1.92 -1.28
AE(HLC) -43.17 -43.17 -43.17 -43.17 -43.17 -24.75 -24.75 -24.75 -30.70 -30.70 -30.89 -30.89
AE(ZPE) +8.54 +19.30 +9.10 +18.08
+19.26 +4.03 +14.17 +14.90 +4.96 +12.41 +4.52 +13.47
0 1993 American Chemical Society
E(G 1 ) -167.681 15 -167.946 06 -167.782 35 -168.029 01 -167.979 08 -92.583 36 -92.770 72 -92.817 33 -113.177 22 -1 13.339 48 -129.739 14 -129.912 36
Proton Affinity of CNO
The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 4615
TABLE 11: C2 Energies and Corrections’ molecule CNO HCNO’ NCO (I
A -8.08 -8.86 -7.48
E(G2) -167.681 25 -167.946 94 -167.781 86
molecule HNCO’ NCOH’ CN
A -9.75 -8.93 -3.96
E(G2) -168.03078 -167.980 03 -92.582 75
molecule HCN+ HNC+ CO
A -5.64 -5.82 -5.97
E(G2) -92.771 80 -92.818 59 -113.177 50
molecule COH’ NO NOH’
A -8.06 -6.53 -8.06
E(G2) -113.341 83 -129.739 97 -129.914 72
Energies in hartrees, corrections in millihartrees.
TABLE III: Harmonic Vibrational Frequencies molecule
harmonic vibrational freq, cm-I
E(HF/6-31Gt), hartrees
CNO HCNO+ CNOH’ NCO HNCO+ NCOH+ c-HCON+
-167.023 -167.306 -167.244 -167.128 -167.388 -167.334 -167.278
78 09 49 87 07 76 63
u 389 u 473 a’ 343 u 555 a’’ 505 a’ 464 a’ 744
u 1179
7811 a” 364 u 1406 a’546 a’’ 509 a’ 959
TABLE Iv: Proton Affinities. G1
protonated species
kJ mol-I
eV
HCNO+ HNCO’ NCOH+ HCN+ HNC+ COH+ NOH+
695.5 647.6 516.5 491.9 614.3 426.0 454.8
7.21 6.71 5.35 5.10 6.37 4.42 4.71
G2 kJ mol-’ 697.6 653.5 519.0 496.4 619.2 431.5 458.8
eV 7.23 6.77 5.38 5.14 6.42 4.47 4.76
expt,b kJ mol-I 669 52lC 595c3d 427e 474
*
a Relative to CNO/NCO, CN, CO, or NO. Using of values from ref 10 unless otherwise stated. Using AHfo(CN) from ref 1 1 . Using AHro(HNCt) from ref 12. eUsing AHro(COH+) from ref 12.
much smaller than for CNO. The correction to the MP4/6311G** ZPVE value is 24.6 kJ mol-’. Of this, 16.8 kJ mol-l comes from the M(2df) correction. The G2 correction is larger for NCO than CNO. The calculated proton affinities of CN, CO, and NO are compared with experimental values at 298 K in Table IV. The experimental values tend to be not known accurately. For example, the values for protonation leading to HCN+, HNC+, and COH+ from the NBS tableslo are 518, 558, and 456 kJ mol-’, respectively, which are significantly different in the case of both HNC+ and COH+ than the values given in Table IV based on new data. We have used revised values for AHo,-(CN) from BerkowitzIO and AHof(COH+)and AHof(HNC+) from Petrie et a1.’2 The effect of varying the temperature from 0 to 298 K is much smaller than the difference observed between theory and experiment. Increasing the temperature to 298 K lowers the calculated proton affinities by 4.3, 5.3, 6.7, and 5.0 kJ mol-’ for HCN+, HNC+, COH+, and NOH+, respectively. The use, where possible, of PMP4 energies rather than UMP4 energies in a modified G1 procedure made a negligible difference in the calculated proton affinities. For NO the E(G1) is -454.9
+
u 2245 ~1117 a’ 1086 u 1958 a’ 871 a’ 1196 a”992
ZPVE, kJ mol-’
02291 a’ 1537
~3512 a’ 2274
a’ 3852
a’ 1240 a‘ 1288 a’ 1235
a’ 2104 a’2310 a’ 1755
a’ 3620 a”3200 a’3427
25.1 56.8 56.6 26.8 53.2 56.6 54.5
kJ mol-’, and for HNCO+ E(G1) is -647.9 kJ mol-’ with PMP4 energies. Even when the uncertainties in the experimentalvalues are taken into account, agreement between theory and experiment for CN or NO is not as good as the 0.045 eV (4.4 kJ mol-’) claimed for the G2 method.’ The calculations on the diatomic molecules suggest that there could be an error in the calculated proton affinities for CNO and NCO by up to 0.25 eV (25 kJ mol-I). Part of the problem with CN is related to the use of a UHF wave function for CN. S2is 1.127 from a UHF/6-31G* calculation on CN. The problem is not as severe for CNO or NCO, S2being calculated to be 0.793 and 0.833 for CNO and NCO in UHF/6-3 lG* calculations. The uncertainty given above is therefore likely to be somewhat pessimistic. The calculated proton affinities of 7.2 eV for CNO and 6.8 eV for NCO should be accurate to f O . l eV.
References and Notes (1) Snyder, L. E.; Buhl, D. Nature (London),Phys. Sci. 1973,243,45. (2) Neyad, J. A. M.; Williams, D. A,; Charnley, S. B. Mon. Nor. R. Astron. SOC.1990, 246, 183.
( 3 ) Hop, C. E. C. A.; van den Berg, K.-J.; Holmes, J. L.; Terlouw, J. K. J . Am. Chem. SOC.1989, 1 1 1 , 72. (4) Bogan, D. J.; Hand, C. W. J. Chem. Phys. 1971, 75, 1532. ( 5 ) Koch, W.; Frenking, G. J. Phys. Chem. 1987, 91, 49. (6) Poppinger, D.; Radom, L.;Pople, J. A. J. Am. Chem. SOC.1977,99, 7806. (7) Curtiss, L. A.; Raghavachari, K.; Trucks, G. W.; Pople, J. A. J. Chem. Phys. 1991, 94, 7221. (8) Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.;
Schlegel, H. B.; Raghavachari, K.; Robb, M.; Binkley, J. S.; Gonzalez, C.; DeFrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.;Kahn, L. R.; Stewart, J. J. P.; Topiol, S.; Pople, J. A. Gaussian 90, Revision F; Gaussian Inc.: Pittsburgh, PA, 1990. (9) Pople, J. A.; Head-Gordon, M.; Fox, D. J.; Raghavachari, K.; Curtiss, L. A. J. Chem. Phys. 1989, 90, 5622. (10) Lias, S. G.: Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. ReJ Data 1988, 17 (Suppl. No. 1). (11) Personal communication from J. Berkowitz to E. E. Ferguson. (12) Petrie, S.; Freeman, C. G.; Meot-Ner, M.; McEwan, M. J.; Ferguson, E. E.J . Am. Chem. SOC.1990, 112, 121.
