J . Phys. Chem. 1991, 95,9782-9787
9782
H,NO,+ Ions in the Gas Phase. A Mass Spectrometric and Post-SCF ab Initlo Study Giulia De Petris,*f Annito Di Marzio: and Felice Grandinetti*J Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Universith La Sapienza di Roma, P . le A. Moro, 5, 00185 Rome, Italy, and the Istituto di Chimica Nucleare del CNR. Area della Ricerca di Roma, C.P. IO, 00016 Monterotondo Stazione, Rome, Italy (Received: May 2, 1991)
Gaseous H2N02+and CH4N02+ions have been obtained by chemical ionization (CI), at pressures ranging from 0.2 to 0.4 Torr. Various reactions have been employed for the preparation of the H2N02+ions, e.g., addition of water to nitrosonium ion, NO+, and protonation of ethyl nitrite and nitroethane by AH+ Bransted acids (A = H2 or CH4). The CH4N02+ions have been obtained by addition of methanol to nitrosonium ion and by protonation of methyl nitrite by AH+ (A = CHI or H20). Structurally diagnostic mass spectrometric techniques, e.g., MIKE and CAD spectrometry, have been employed to probe the H2N02+and CH4N02+populations from the above reactions. Strong evidences have been obtained for the existence of two CH4N02+isomers, which have been assigned the (CH,-NOH)+ and (CH30H-NO)+ structures, whereas the single isomer detectable in the H2N02+populations has been identified as the nitrosohydronium ion, (H20-NO)+. The results of molecular orbital calculationsat the MP4(SDTQ)//6-31 lG**/MP2(FU)//&31G** + ZPE (MP2(FU)/6-31G**) post-SCF level of theory identify six different conformers of the H2N02+ion as stable species (true minima on the surface), the nitrosohydronium ion being the most stable one. At the post-SCF level of theory, the latter species is viewed as an ion-molecule complex between NO+ and H20, rather than as a normal cation. Employing the Gaussian-1 theory, recently outlined by Pople and co-workers, a heat of formation of 160 2 kcal mol-' has been computed for this ion,which compares fairly well with the experimental heat of formation of the H2N02+ion, 159 1.5 kcal mol-].
*
Introduction Gaseous H 2 N 0 2 +ions are of considerable importance in atmospheric chemistry, being the first link in the chain of consecutive hydration steps that eventually yields nitrous acid and H+(H20), clusters.'-3 The relevance of the process can be appreciated if one considers that NO+ is a major positive ion in the D region, being the main product from the natural and the man-made ionization of air? and that its hydration contributes to nitrogen fixati~n.~ In solution, protonated nitrous acid plays a fundamental role in the nitrosation reaction," whose kinetics and mechanism have been extensively investigated in various experimental conditions. In particular, it has been demonstrated that the rate-determining step of the nitrosation process at high acidity involves an attack either by free nitrosonium ion, NO+, or by its hydrated form, H2N02+.6 Further, the reactions of this ion with sulfur, oxygen, and nitrogen nucleophiles are the subject of considerable interest, especially as a route to nitros~amines.~J~ Direct evidence for the protonation reaction of nitrous acid by a Bronsted acid HN02
+ BH+
+
H2N02'
+B
(1)
as a route to the formation of H2N02+ions is not easily gained in the gas phase, due to the fact that nitrous acid can hardly be obtained in the pure state, being generally accompanied by its dissociation products, NO, NO2, and water vapor, together with N204, N203,and nitric acid." At low pressures (ca. 1 Torr) and room temperature, nitrous acid accounts for less than about 0.3% of the total mixture." As a consequence, previous experimental studies of gaseous H2N02+ions are restricted to its formation via hydration of NO+.2*'2 Not unexpectedly, the adduct from the process H20
+ NO+
-
H2N02+
(2)
has been assigned the H20-NO+ structure.lh*b The available thermochemical information is limited to a single measurement of AHo2 = -18.5 f 1.5 kcal mol-', reported as early as in 1973 by Kebarle and From this value, one derives a heat of formation of H2N02+of 159 kcal mol-', corresponding to a proton affinity (PA) of nitrous acid of 187.7 kcal mol-', in line with those of alkyl nitrites, RONO, that increase regularly from 192.5 (R = CH3) to 205.7 (R = t-C4H,) kcal m01-l.13 'University of Rome "La Sapienza". Istituto di Chimica Nucleare del CNR.
*
0022-3654/91/2095-9782%02.50/0
*
To our knowledge, no further experimental thermochemical data on H2N02+,except a AHo2value from a high-temperature flame-ionization study,lZChave since been reported; nor has the existence of other isomers, e.g., N(OH)2+and ON(H)OH+, been investigated by structurally diagnostic mass spectrometric techniques. As to the theoretical approaches, whereas their results consistently identify the most stable H2N02+protomer with species broadly classifiable as nitrosohydronium ions, H20-NO+, the calculated stability and geometry of the latter, especially as regards the H20-NO+ separation, vary quite considerably, depending on the computational methods employed, providing contrasting pictures of the structure and the bonding of gaseous H2N02+,and hence of the PA of nitrous acid. Earlier studies, based on the semiempirical MNDO method,I4 and on limited basis set ab initio c a l c ~ l a t i o n s ,characterized ~~~*~ the most stable H2N02+ion as a bona fide hydrated nitrosonium ion, with a H2CbNO+separation comparable to the HO-NO bond distance in HN02, i.e., sufficiently small to allow significant orbital overlap. The calculated heat of formation of H2N02+corresponds to a PA of nitrous acid (1) Fehsenfeld, J. C.; Ferguson, E. E. J . Geophys. Res. 1969, 74, 2217. (2) Fehsenfeld, J. C.; Mostsman, M.; Ferguson, E. E. J. Chem. Phys. 1971, 55, 2120, and references therein. (3) Ferguson, E. E. In Kinetics of Ion-Molecule Reactions; Ausloos. P., Ed.;Plenum: New York, 1979; p 377, and references therein. (4) Landshoff, R. Thermal Radiation Phenomena; Lockheed Missile and Space Co.: Sunnyvale, CA, 1967; Vol. 4. (5) Ferguson, E. E.; Libby, W. F. Nature 1971, 229, 37. (6) Patai, S.,Ed. The Chemistry of Nitro and Nitroso Groups; Wiley: New York, 1970; Parts I and 2. (7) . . (a) . . Williams. D. L. H. Ado. Phvs. Om. Chem. 1978.16.159. (b) Ridd, J. H. Adu. Phys. Org. Chem. 1978, i6, I,&d references therein. (8) Jsrgensen, K. A.; Lawesson, S. 0. J . Chem. Soc., Perkin Trans. 2 1985, 231. (9) Aldred, S. E.; Williams, D. L. H.; Gasley, M. J . Chem. SOC.,Perkin Trans. 2 1982, 11. (IO) Magee, P. N.; Barnes, J. M. Adu. Cancer Res. 1967, 10, 163. (11) Cox, A. P.; Kuczkowski. R. L. J . Am. Chem. Soc. 1966,88, 5071, and references therein. (12) (a) Fehsenfeld, F. C.; Howard, C. J.; Schmeltekopf, A. L. J . Chem. Phys. 1975, 63, 2835. (b) French, M. A,; Hills, L. P.; Kebarle, P. Can. J . Chem. 1973, 51, 456. (c) Burdett, N. A.; Hayhurst, A. N. J . Chem. Soc., Faraday Trans. 1 1982, 78, 2991. (13) From the data of (a) Farid, N.; McMahon, T. B. Inr. J . Mass Spectrom. Ion Phys. 1978,27, 163; adjusted to the current relative PA scale by: (b) Lias, S.G.;Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levine, R. D.; Mallard, W. G.J . Phys. Chem. Ref, Data 1988, 17, Suppl. No. 1 . (14) Dewar, M. J . S.;Shanshal, M.; Worley, S. D. J . Am. Chem. SOC. 1969, 91, 3590. ( 1 5) (a) Dargelos, A.; El Ouadi, S.;Liotard, D.; Chaillet, M.; Elguero, J. Chem. Phys. Lett. 1977, 51, 545. (b) Edwards, W. D.; Weinstein, H. Ibid. 1978, 55, 582. (c) Nguyen, M. T.; Hegarty. A. F. J . Chem. Soc., Perkin Trans. 2 1984, 2037.
0 1991 American Chemical Society
H 2 N 0 2 +Ions in the Gas Phase
The Journal of Physical Chemistry, Vol. 95, No.24, 1991 9783
below 190 kcal mol-', fairly consistent with the values from the experimental studies.2J2 More recent a b initio studies, at the 4-3 1G S C F level of theory, present a different picture of gaseous H2N02+,viewed as an ion-dipole complex where significant orbital overlap between NO+ and H 2 0 is prevented by their large separation, 1.9688 to 2.128 &ISC and the calculated PA of H N 0 2 is as high as 203.3 kcal As a whole, the available set of data on the structure, the bonding, and the heat of formation of the H2N02+ions is surprisingly limited, in view of their important role in atmospheric and organic chemistry, which suggests further investigation of the matter, particularly on account of the evidence pointing to the existence of two gaseous protomers of methyl nitrite, characterized by the (MeOH-NO)+ and the (MeONOH)+ structure (vide infra). We present here the results of an experimental and theoretical study of H2N02+,based on various mass spectrometric techniques, Le., chemical ionization (CI), metastable ion kinetic energy (MIKE), and collisionally activated dissociation (CAD) mass spectrometry, complemented by M O ab initio calculations, a combination which has proved useful in the study of the closely related protonated nitric acid.16 The Gaussian-] theory, as recently outlined by Pople and co-workers," has been employed for an accurate evaluation of the energetics of reaction 1. The specific objectives of the present investigation are, first, to explore new routes to gaseous H2N02+ions, second, to probe the H2N02+populations from different reactions with structurally diagnostic techniques in the search of isomeric species, and, finally, to compare the currently available values of the heat of formation of H2N02+'2bwith an accurately computed one, in order to unequivocally assign the structure of the experimentally observed H2N02+isomer.
TABLE I: CAD Spectra of H2N02+ Ions from Different Reactions
Experimental Section Materials Nitric oxide from Aldrich-Chemie GmbH, methane, helium, and hydrogen from Sol Sud Co. were high-purity gases, used without further purification. The ethyl nitrite obtained from the same source as a 15% solution in ethanol, was purified by GLC on a 2.5-m column packed with Apiezon-L, working at a temperature of 40 "C. Methyl nitrite was prepared from N a N 0 2 , methanol, and sulfuric acid, trapped with liquid nitrogen, and purified with vacuum techniques in a greaseless system. Mass Spectrometric Measurements. Mass spectrometric measurements were performed on a VG ZAB-2F instrument operating in the chemical ionization (CI) mode. Typical experimental conditions were source temperature 180 "C, emission current 0.5 mA, repeller voltage 0 V, reactant gas pressure 0.2-0.4 Torr. MIKE spectra were recorded at an energy resolution ranging from 4 X IO3 to 8 X IO3 and represent the average of at least 40 scans. CAD spectra were taken at a lower energy resolution, by admitting He into the collision cell at such a pressure to reduce the main beam intensity to 30% of its initial value. Computational Details. Ab initio quantum-mechanical calculations were performed by running a IBM/VM-CMS version of the GAUSSIAN 88 set of programs.18 The standard internal 631G**,'" 6-31 IG**,lgb6-31 I+G**,'* and 6-31 1G**(2df)'* basis sets were thoroughly employed. Geometry optimizations were performed, in the full space of the coordinates, by analytical gradient based techniques,20 in the framework of the Mlaller-
Mass Spectrometric Results Different Routes to the H2N02+and CH4NO2' Ions. Various reactions have successfully been employed for the preparation of gaseous H2N02+ions, e.g., the addition H2O NO+ H2N02' (2) which occurs efficiently under typical CI conditions. Using H2I80, reaction 2 yields labeled ions that, significantly, incorporate a single I8Oatom. Another useful precursor has been found in ethyl nitrite, which gives H2N02+as a charged fragment following highly exothermic protonation by H3+or CHS+: CZHsONO + HA+ H2N02' + C2H4 + A (3) where A = H2 or CH4 This process represents an additional example of a general class of reactions promoted by protonation of ethyl esters of inorganic acids, e.g. C2HSON02Uand C2HSN?.24 An additional precursor has been found in nitroethane, which gives a H2N02+fragment by proton transfer from H3+or CHS+ ions: CZHSONO + HA+ H2N02' C2H4 A (4) where A = H2 or CHI. The CH4N02+ions have been obtained by the solvation process C H 3 0 H + NO+ CH4N02+ (5) and by proton transfer to methyl nitrite from acids of different strength: CHSONO + HA+ CH4N02' A (6) where A = CH4 or H 2 0 . MIKE and CAD Spectrometry. A general feature of the H2N02+populations is that their MIKE and CAD spectra show no significant differences, irrespective of the formation process and of its energetics.
reaction 2 (H20) 2 (H2I80) 3 (A = H2) 3 (A = CHI) 4 (A = H2)
re1 intensities of charged fragmentsu mfz = 8 m f z = 20 m / z = 30 11 89 10 90 10 90 12 88 8 92
OThe intensities are normalized respect to the sum of the intensities.
Plesset theory,21truncated at the second order, with the 6-31G** basis set. The MP2 theory was used with full (FU) electron correlation (including inner-shell electrons). The geometries obtained in this way are denoted by MP2(FU)/6-31G**. The unscaled MP2(FU)/6-31G** vibrational frequencies were thoroughly employed for all of the H N 0 2 and H2N02+found critical points, in order to characterize them as true minima or higher order saddle points on the corresponding potential energy hypersurfaces. The zero-point energies (ZPE's) of all the investigated species were accounted for in this way, and the thermal corrections to the ZPE's were made by assuming a 289.15 K temperature. The absolute entropies of the H N 0 2 and H2N02+isomers were also obtained by using the above-computed frequencies. Single-point calculations, a t the post-SCF level of theory, were performed with the Mlaller-Plesset theory up to the fourth order (MP4), by including single, double, triple, and quadruple excitations. A post-MP4 correction for residual correlation energy contributions was accounted for by quadratic configuration interaction, QCISD.22 The Gaussian- 1 procedure, as outlined in ref 17, was employed to obtain thermochemical data for the investigated processes.
-
+
-
+
+
-
+
(16) Cacace, F.; Attini, M.; de Petris, G.; Speranza, M.J . Am. Chem. Soc. 1990, 112, 1014. (17) Pople, J. A.; Gordon, M. H.; Fox, D. J.; Raghavachari, K.; Curtiss, L. A. J . Phys. Chem. 1989, 90,5622. The MP2(FU)/6-31G** geometries have been employed for the single-point calculations, which have to be performed in the framework of the Gaussian-I theory. (18) Frisch, M. J.; Head-Gordon, M.;Schlegel, H. B.; Raghavachari, K.; 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.; Fluder, E. M.; Topiol, S.; Pople, J. A. GAUSSIAN 88; Gaussian Inc.: Pittsburgh, PA, 1988. (19) (a) Hariharan, P. C.; Pople, J. A. Chem. Phys. Lett. 1972, 66, 217. (b) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Ibid. 1980, 72,4244. (c) Frisch, M. J.; Pople. J. A.; Binkley, J. S.J . Chem. Phys. 1984,80, 3265. (20) Schlegel, H. B. J . Compuf. Chem. 1982, 3, 214.
+
+
(21) Molter, C.; Plesset, M. S. Phys. Reo. 1934, 46, 618. (22) Pople, J. A.; Head-Gordon, M.;Raghavachari. K.J . Chem. Phys. 1981, 87, 5968. (23) Nixon, W. B.; Bursey, M. M.Tetrahedron 1970, 50, 4389. (241 (a) Attinl. M.:Cacace. F.: de Petris. G.: Grandinetti, F. Int. J. Mass Spectrom. Ion Processes 1989,!?0, 263. (b) Cacace, F.; AttinH, M.; de Petris. G.; Grandinetti, F.; Speranza, M. Guzz. Chim. Itul. 1990, 120, 691.
;D[
de Petris et al.
9784 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
a
ti
0
In&
IRa
(la)
(Ib)
I
IB
I
rl
B
II
Figure 1. (a) Unimolecular decomposition of C H 4 N 0 2 +ions from reactions 5 and 6 (A = H 2 0 ) into NO+ and CH,OH; (b) unimolecular
decomposition of CH4N02+ions from reaction 6 (A = CH,) into NO+ and CHIOH. TABLE II: CAD Spectra of CHAN02+Ions from Different Reactions
re1 intensities" reaction 6 mlz
reaction 5
41 46 45 32 31 29
3.6 23.9 48.0 21.8 2.6
A
H20 2.0 18.3 54.5 22.4 3.0
A = CHI
4.1 9.1 10.9 6.5 39.9 22.1 1.2
IS "The intensities are normalized respect to the sum of the intensities. The peak of mass m / z 30 has been excluded because of its unimolecular contribution.
In the first place, the H2N02+ions consistently fail to undergo metastable decomposition, giving blank MIKE spectra. Secondly, their CAD spectra are characterized by two charged fragments, Le., the H 2 0 + and NO+ ions, at m/z 18 and 30. Furthermore, H2N02+ions from reactions 2-4 give indistinguishable CAD spectra whose differences fall within the range of the experimental error, as shown in Table 1. Significantly, the CAD spectrum of the H2N0I80+ions from process 2 carried out using H2I80as a reactant is characterized by the exclusive loss of H2180,strongly suggestive of a (H2'*0-NO)+, rather than of a (HI80-N-OH)+ structure. In contrast, the mass spectrometric study of CH4N02+ions from reactions 5 and 6 provides strong evidence for the existence of two isomeric structures. In fact, reactions of low exothermicity, i.e. (5) and (6) (A = H20), give ionic populations whose MIKE spectra are characterized by the metastable NO+ fragment with a Gaussian-type peak, as shown in Figure la, and whose CAD spectra display major charged fragments at m/z 29, 3 1, and 32, in addition to a NO+ peak, at m/z 30, which contains a unimolecular component. On the contrary, populations of CH4NO2+ ions, which contain a considerable excess of internal energy, due to the greater exothermicity of their formation process (reaction 6, in low-pressure methane) give MIKE spectra characterized by a NO+ fragment with composite peak, where a Gaussian-type component is superimposed on a wider peak or dish-topped peak, with a higher kinetic energy release (see Figure 1b). The corresponding CAD spectra are also markedly different from those of the populations from low-energy reactions, the fragment at m / z 15 being more abundant and the one at m/z 32 considerably less abundant and two additional fragments, at m/z 45 and 46, being present (see Table 11). Computational Results Structure and Energetics of HzN02+. Ab initio calculations on the structural and energetic features of various H2N02+isomers have been performed at the post-SCF level of theory. A prelim-
inary study of the HNO, molecule has been carried out, at the same computational level, as required for a careful evaluation of the energetics of the protonation reaction 1 of nitrous acid. Geometry optimizations, at the MP2(FU)/6-31G** level of theory, have been performed for the cis- and trans-nitrous acid. Harmonic vibrational frequencies, at the same computational level,
I103
N-0
0. 103.9
\' H
Figure 2. Optimized geometries of the two la and l b conformers of the nitrosohydronium ion, of the nitrosonium ion, and of the water molecule, at the MP2(FU)/6-31G** level of theory. Bond lengths are in angstroms and bond angles in degrees.
have been computed for both isomers, in order to include zero-point corrections in the calculation of the relative stability of the two forms. The optimized geometries of the two H N 0 2 isomers are collected in Table 111. Previously obtained theoretical geometries25 and the experimental oneZsaare also reported in the same table for comparison. It is apparent that substantial agreement does exist between the recently published MP4(SDQ)//6-311G** p a r a m e t e r ~ ~and ~ g the present ones computed a t the MP2(FU)/6-31G** level, especially with regard to the bond angles. Further, both sets of values are in very good agreement with the experimental findings, suggesting that the degree of inclusion of the electron correlation energy effects is not crucial for a careful evaluation of the skeletal bond angles in this kind of molecules. On the contrary, as far as the theoretical estimate of the bond lengths in the HNOZ molecule is concerned, the data collected in Table 111 show that the electron correlation energy effects have to be carefully taken into account for an accurate evaluation of these parameters. As a matter of fact, the experimental value of the length of the double N=O bond is best reproduced at the MP4(SDQ)/6-311G** level of theory:sg whereas a more accurate value of the distance of the single N-0 bond is obtained at the MP2( FU)/6-3 1G** level of calculation employed in this work, and a comparable deviation (0.02-0.03 A) from the experimental values characterizes both of the computational levels. In any case, it should be underlined that the differences between the absolute experimental values of the cis- and trans-nitrous acid geometrical parameters are fairly well reproduced both at the MP2(FU)/631G** and at the MP4(SDQ)/6-31G1* level of theory. Structure and Energetics of the H2NO2+ Conformers. As previously pointed out, the study of the protonated forms of nitrous acid has been addressed in the past years by ab initio molecular-orbital calculations, which have identified six different conformers of the H2N02+ion at the S C F level of theory and have allowed an evaluation of their structural and energetic features.8+'s However, relatively small basis sets have been used in order to obtain the corresponding optimized geometries, and the electron correlation energy effects on the structures and the relative stabilities of these isomers have not been discussed. Further, some of the investigated H2N02+forms have been found to have very close stabilities,'% whose order could substantially be affected by the effects of zero-point energies, not accounted for in the previous investigations. Furthermore, the computation of the harmonic vibrational frequencies affords the possibility of checking whether all of the H2N02+critical points identified correspond to true minima (stable isomers), transition states or higher order saddle points on the potential energy surface. (25) The results of previous theoretical investigations of the structure and the energetics of neutral nitrous acid are provided in the following papers: (a) Skaarup, S.; Boggs, J. E. J . Mol. Srrucr. 1976, 30, 389. (b) Turner, A. G. J . Phys. Chem. 1985,89,4480. (c) Darsey, J. A,; Thompson, D. L. Ibid. 1987, 91, 3168. (d) Wiberg, K. B. Inorg. Chem. 1988,27,3694, (e) Toro-LabW, A. J. Mol. Struct. (THEOCHEM) 1988, 180,209. (f) Suter, H. U.; Huber, J. R. Chem. Phys. Lerr. 1989,155,203. (8) Wiberg, K. B.; Breneman, C. M.; LePage, T. J. J . Am. Chem. SOC.1990, I12, 61. (h) Coffin,J. M.;Pulay, P. J . Phys. Chem. 1991, 95, 1 1 8.
The Journal of Physical Chemistry. Vol. 95, No. 24, 1991 9785
H2N02+Ions in the Gas Phase
TABLE III: Experimental and Theoretical Geometries, at Various Computational Levels, for the Cis and Trans Isomers of Nitrous Acid
species cis- H N O2
trans- H N O2
parameter'
CISDb
CEPA- 1
MP2/ 6-3 1G** '
MPWDQ)// 6-311G**d
exptC
R(N=O) R(N-0 R(0-H) a(O=N-0) a(N-0-H) R(N=O) R(N-0) R(0-H) a(O=N-0) a(N-0-H)
1.20 1.37 0.98 112.8 105.2 1.19 1.40 0.97 110.7 102.6
1.20 1.39 0.98 112.7 104.6 1.19 1.43 0.97 110.5 101.7
1.209 1.385 0.98 1 112.8 104.4 1.196 1.423 0.97 1 110.3 101.7
1.186 1.376 0.973 113.1 105.0 1.175 1.405 0.963 110.6 102.1
1.185 1.392 0.982 113.7 104.0 1.170 1.432 0.958 110.7 102.1
"Bond lengths (R) are in Angstroms and bond angles (a)in degrees. bReference 25f. 'This work. "Reference 25h. CReference28a. TABLE IV: Total Energies (atomic units), ZPE's (kcal mol-'), and Absolute Entropies (cal mol-' the Nitrosonium Ion. NO+. and the H,O Molecule
species la
lb
MP2/6-31G** -205.51063
MP2/6-31GS* MP3/6-31 IG** MP4/6-31 IG** -205.58278 -205.640 72
2
-205.509 92 -205.449 66
-205.582 97 -205.53273
-205.640 33 -205.577 41
3
-205.444 4 1
-205.52699
-205.571 90
4
-205.422 08
-205.503 48
-205.549 30
5
-205.439 70
-205.51999
-205.565 27
6
-205.450 40
-205.527 27
-205.574 54
H2O
-76.222 45
-76.268 32
-76.276 27
NO+
-1 29.248 I7
-129.27841
-129.32641
K-l)
for the 1-6 Isomers of the H2N02+Ion,
ZPE (MP2/6-31G**) 18.8 20.1 (298.15)
5 (MP2/6-3 1G**) 67.7
21.7 21.9 (298.1) 21.5 21.7 (298.15) 21.0 21.2 (298.15) 21.5 21.8 (298.15) 21.5 21.7 (298.15) 13.7 13.8 (298.15) 3.03 3.03 (298.15)
57.7 59.2 58.1 59.4 59.2 45.1 47.5
TABLE V Calculated Relative Stabilities for the 1-6 Isomers of the H2N02+Ion' AH'(298.15). kcal mol-'
species la 2
3 4 5 6
MP2/6-3 1G** 0 +39.4 +40.1 t43.2 t46.2 +56.7
MP3/6-31 lG**// MP2/6-31G** 0 +36.4 +33.2 +36.6 +41.1 +50.9
MP4/6-311G**// MP26-3 lG** 0 +41.7 +41.5 +44.8 +49.0 +58.5
TAS
AG0(298.15), kcal mol MP4(SDTQ)/6-31 IG**
0 -2.5 -3.0 -2.5 -2.5 -2.9
+44.2 +44.5 +47.3 +51.5 +61.4
0
'The data are expressed as the values of the thermodynamic functions of the equilibrium H2N02+(la) F! H2N0,+(X) (X = 2-6). Optimized geometries, a t the MP2(FU)/6-31G** level of theory, for seven different conformers of the H2NOZ+ion are reproduced in Figures 2 and 3. The absolute energies of these species, at various computational levels, are collected in Table IV, and their energies relative to the most stable form are reported in Table V; the zero-point energies and the absolute entropies of isomers 1-6 are summarized in Table IV. In agreement with previous theoretical concIusions,8Jk the data of Table V show that the most stable isomer of protonated nitrous acid is obtained when the protonation occurs at the oxygen atom of the hydroxylic moiety, yielding the nitrosohydronium ion 1 whose MP2(FU)/ 6-31G** structure is shown in Figure 2. It seemed important to verify whether inclusion of correlation-energy effects could substantially modify the SCF structure of this which was previously described as an ion-molecule complex between the nitrosonium ion, NO+, and a water molecule rather than a normal cation. The present structural results reveal that this kind of description holds at the post-SCF level of theory, which points to a very long separation (2.204 A) of the NO+ and HzOmoieties, even larger than the SCF computed one (2.1 28 .A8). The structural features of these two fragments are not substantially different from those computed for the free NO+ and HzO, whose MP2(FU)/ 6-31G** geometries are also shown in Figure 2 for comparison.
The detailed investigation of the HzN02' potential energy surface in the region of the nitrosohydronium ion revealed two critical points, corresponding to ions l a and l b of Figure 2. The first one (la) possesses a symmetrical, nonplanar structure (C, symmetry), and the second one is a fully planar system. Calculation of the corresponding harmonic vibrational frequencies shows that the nonplanar form is a true minimum on the potential energy surface, but a low imaginary frequency is associated to the planar conformer. Examination of the transition vector corresponding to this negative eigenvalue of the Hessian matrix revealed that ion l b must be viewed as the transition state, which connects two symmetrical forms of ion la, through an inversion process at the oxygen atom. As shown in Table IV, the absolute energies of ions l a and l b are very close, so that the inversion process must be considered as practically free. These observations reverse earlier c o n c l ~ ~ i o concerning n s ~ ~ ~ the nature of the planar and nonplanar forms of ion 1. In fact, at the S C F level of theory, the planar form was recognized as the most stable one, and the nonplanar isomer was described as a rotational transition state. We find that la and lb have a comparable stability but different geometrical features (see Figure 2), the long H20-NO+ bond distance differing by 0.030 A, against an uncertainty of ca. 0.020 8, associated to the MP2(FU)/6-3 lG**computed bond distances
9786 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
de Petris et al. 42 to ca. 58 kcal mol-' does exist between isomer la and 2-6.
H
(3)
(2)
H
(6) Figure 3. Optimized geometries of the 2-6 H2N02+isomers, at the MPZ(FU)/6-3lG** level of theory. Bond lengths are in Angstroms and bond angles in degrees.
of small molecules.26 Further, the O N 0 angle markedly varies on passing from isomer l a to conformer lb. The geometries of the isomers 2-6 of protonated nitrous acid optimized at the MP2(FU)/6-31G** level of theory are reported in Figure 3. As a general observation, the harmonic vibrational frequencies calculations revealed that all of these isomers correspond to minima on the MP2(FU)/6-31G** H2N02+potential energy surface; i.e., all of them can exist, at least in principle, as stable species. Both isomer 2 and isomer 6 correspond to nitrogen protonated forms of nitrous acid, while 3,4, and 5 are obtained by protonation of the oxygen atom of the N 4 moiety in HNOz (Figure 3). The data of Table V show that, at the MP4(SDTQ)/6-31 1GS*//MP2(FU)/6-31G** level of theory, with inclusion of zero-point energy corrections, isomers 2 and 3 are practically degenerate, the former being slightly favored when the entropy effects are also included. This conclusion substantially modifies the S C F order of stability for the H2N02+ isomers previously reported,'" identifying the nitrosoacidium ion 3 as more stable than isomer 2. lsomers 4 and 5 can be formally obtained by a sequence of 1 80° rotations of the two O-H bonds of isomer 3. From the data reported in Table V, the form 5 can be observed to be less stable than 3 by ca. 7 kcal mol-', and this difference can be probably ascribed to the repulsive interaction between the two hydrogen atoms in cis position, which is only partially counterbalanced by the formation of two intramolecular hydrogen bonds. In the form 4, whose stability is intennediate between those of 3 and 5, only one intramolecular hydrogen bond can be formed, but there are no strong repulsive interactions between the hydrogen atoms. ln this perspective, the high energy difference, ca. 17 kcal mol-', between isomers 2 and 6, the latter being the less stable one, can be ascribed to the steric repulsion between the two hydrogen atoms. Intramolecular hydrogen bonds cannot be formed in isomer 6, and hence, the repulsive interaction cannot be counterbalanced by the attractive ones. As a final remark, the data of Table V show that a large energy gap ranging from ca. (26) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (27) de Petris, G. Org. Mass Speerrom. 1990, IO, 557. (28) (a) Finnipn. D. J.; Cox, P. A.; Brittain, A. H.; Smith, J. G. J . Chem. Soc., Faraday Trans. 2 1972,68,548. (b) Varma, R.; Curl, R. F.J . Phys. Chem. 1976,80, 402.
Discussion The mass spectrometric results previously illustrated show that a single isomer is detectable in the H2N02+populations from reaction 2 to 4. This conclusion rests on the following circumstantial but mutually reinforcing pieces of evidence, suggesting that a single species, corresponding to the most stable H2N02+ isomer, of the nitrosohydronium structure is formed from reactions 2-4: (i) the CAD spectra display only the NO+ and H20+ fragments; (ii) the isotopic exchange which incorporates a single I8Oatom between H2N02+and H2180yields a labeled ion, whose CAD spectrum displays only the NO+ and H2180+fragments. In principle, the above results could be accounted for by (i) the lack of structural discrimination by the mass spectrometric techniques employed; (ii) the exclusive formation of a single H2N02+isomer from reactions 2-4; (iii) the fast interconversion, within the sampling time ( s), of those H2N02+isomeric ions with a large content of internal energy, which survive decomposition in the ion source, into a single most stable structure. The first hypothesis finds no support in the comparison with the strictly related CH4N02+system, where MIKE and CAD spectrometry detect large differences in the populations of ions from different reactions. Energetic considerations based upon the theoretical results previously described strongly militate against the second hypothesis, since formation of all H2N02+isomers is energetically allowed from reactions 3 (A = H2) and 4 (A = H2).B Strong evidence for the existence of two CH4N02+isomers is provided by the following observations: (i) a fragment corresponding to the loss of a neutral OH group ( m / z 45) is detected from CH4N02+ions from highly exothermic reactions, suggesting its presence in the ions being assayed consistent with the structure (CH,O-NOH)+; (ii) the fragment at m / z 32 is remarkably abundant in the CAD spectrum of ions from reactions 5 and 6 (A = H20), suggestive of the presence of the CH30H moiety in the molecule of the ionic precursor; (iii) the unimolecular decomposition into CH30H and NO+ ions of the (CH30-NOH)+ form requires structural rearrangement, which could account for the observed kinetic-energy release. Significantly, strictly related ions, e.g., protonated nitric acidI6 and methyl nitrate,,' have been found to exist in different isomeric forms, depending on the energetics of the formation reactions. Resting on these considerations, the present failure to detect different isomeric structures of the H2N02+ions is consistent with the exclusive formation of the nitrosohydronium ion, la. The stability difference between ion l a and ions 2-6 may be sufficient to prevent rearrangement of l a into the less stable isomers, if the direct formation of the nitrosohydronium ion from reactions 3 and 4 is assumed. Another possibility is that isomers 2 and/or 6, Le., the nitrogen-protonated forms of nitrous acid, are actually formed by reactions 3 (A = H,) and 4 (A = H,) but undergo fast isomerization to the more stable ion la, via a 1,tproton shift, whereas conversion to isomers 3, 4, and 5 is not energetically allowed. Finally, if isomers 3,4, and 5 are initially obtained by reactions 3 and 4, they are expected to undergo fast isomerization to the more stable form 2, and hence to the nitrosohydronium ion, la. As a whole, the experimental and the theoretical results concur in the conclusion that the nitrosohydronium ion is the most stable form among the various H2N02+isomers. The mass spectrometric techniques employed in this study do not allow accurate measurement of the heat of formation of this structurally characterized ion. On the other hand, the only experimentally available heat of formation of the H2N02+ion, 160 kcal refers to a (29) From the experimental heat of formation of isomer 1, 160 kcal mol-', and the calculated heats of formation of isomers 2-6, the energetics of reactions 2-4 can be evaluated as follows (in kcal mol-') (a = isomer 1, b = isomer 6): AHo* = (a) -18.2, (b) +38.5; W , ( A = H3+) = (a) -68, (b) - 1 1 ; AHo3(A= CHS+)= (a) -37.3, (b) +19.4; W , ( A = HI+) = (a) -68.6, (b) -11.9; AHo4(A = CHs+) = (a) -37.9, (b) +18.8. It IS clear that all the isomers can be formed from protonation by H,+, whereas proton transfer by CHI+as well as the addition (2) fail to give rise to the formation of isomers other than 1; in fact, AHo3(A = CHs+), AHo,(A = CHs+), AHo2 = +2.1, +1.5, and +21.2 kcal mol-' for isomer 2.
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 9787
H2N02+Ions in the Gas Phase
TABLE VI: Gaussian-1 Total Energies and Correction Terms for the Cis and Trans Isomers of Nitrous Acid3'
Energies" MP4(STDQ)/6-31 IC**// MP2/6-31G**
AE(+)
AE(2df)
AE(QC1)
Gaussian-I
ZPEb
cis-HN02
-205.309 73
-1 1.47
-109.05
+6.34
-205.423 91
trans-H NO2
-205.308 94
-1 3.95
-108.3
+6.03
-205.425 16
12.8 13.0 (298.15) 12.7 12.9 (298.15)
species
Energy Differences (Etrm,-Ecis)C MP4(SDTQ)/6-3 1 lG**// MP2/6-31GS*
species trans- H NO2 a
W + )
0
0 +0.50
cis-HNOZ
-1.56
AE(2df)
WQCU
0 +0.47
0 -0.19
Gaussian-I 0 -0.78
ZPEb 0
-0.10
Energies in hartrees; corrections in millihartrees. kea1 mol-I. Ckcalmol-'.
not structurally characterized species. These considerations have suggested to attempt an accurate theoretical evaluation of the heat of formation of the nitrosohydronium ion and to compare the results with the experimental one. To this end, the calculation of the nitrous acid PA has been performed by high-level ab initio techniques, assuming formation of ion la. The Gaussian-I theory, a general predictive computational procedure recently developed by Pople and co-worker~,'~ which seems to be able to reproduce known thermochemical data to a target accuracy of f 2 kcal mol-', has been selected for this evaluation. In the framework of this theory, which can be considered a posr-hfP4 one, the correction for residual electron correlation energy effects, not taken into account at the MP4(SDTQ) post-SCF level of theory, is introduced by quadratic configuration interaction (QCISD) which is known to reproduce full configuration interaction (FCI) results quite well, particularly near equilibrium geometriesz To avoid very extensive calculations at the QCISD(T)/6-311+G**(2df) level of theory, four correction terms are added to the MP4(SDTQ)/6-31 lG**//MP2(FU)/63 IC* absolute energies, accounting respectively for the effects of the diffuse sp basis functions, of the higher polarization functions on non-hydrogen atoms, of the corrections for residual correlation effects, and of the so-called higher level correction. Three additional calculations are required in order to perform these corrections, at the MP4/6-31 I+G**, MP4/6-311G**(2df), and QCISD(T)/6-31 IG** level of theory, respe~tive1y.l~ If one denotes as E , , E2, and E3 the corresponding absolute energies, the correction terms to the MP4/6-31 IC** computed value, identified as Eo, are AE(+) = El -Eb AE(2df) = E2 - Eo, and AE(QC1SD) = E3 - Eo, respectively. The final value of the Gaussian-I energy of a species is E(GI) = Eo AE(+) AE(2df) AE(QC1SD) AE(HLC)
+
+
+
+
the fourth correction term, AE(HLC), being introduced in a parametric way.I7 A computational test has been carried out on the strictly related H N 0 2 molecule, calculating in particular the energy difference between its cis and trans isomers. The experimentally measured difference is small and affected by relatively large uncertainties, e.g., values of 0.4 f 0.1 and of 0.5 f 1.O kcal mol-' have been r e p ~ r t e d . ~ *Nevertheless, ~*~~ it is legitimate to conclude that the trans isomer of nitrous acid seems to be energetically favored with respect to the cis one by less than 1 kcal mol-', a subtle difference whose evaluation requires a highly precise theoretical procedure, with a prescribed degree of accuracy. The relevant energetic data for the theoretical evaluation of the energy difference between the two H N 0 2 isomers in the framework of the Gaussian-1 theory are collected in Table VI. The higher level correction terms (AE(HLC)) are not reported, being equal for isomeric species. In the framework of the Gaussian-1 theory, the trans isomer of nitrous acid is found to be more stable than the cis one, the computed energy difference (30) McGraw, G.E.; Bernitt, D. L.;Hisatsune, I. C. J . Chem. Phys. 1966, 45, 1392.
being 0.88 f 2 kcal mol-' when the zero-point corrections are taken into account. Although the uncertainty associated with this estimate is large if compared to the absolute value, the stability order of the isomers corresponds to the experimental one. As apparent from Table VI, the factor which substantially favors the trans isomer of nitrous acid, from the energetic point of view, seems to be the inclusion of diffuse functions on non-hydrogen atoms. The residual correlation energy effects, on the contrary, do not appear very important in determining the value of this energy difference. The results of the test calculations on HNOz have encouraged us to use the Gaussian-1 procedure in the evaluation of thermochemical data for the protonation of nitrous acid. The correction terms to the MP4(SDTQ)/6-31 lG**//MP2(FU)/6-31G1* total energy of the nitrosohydronium ion, la, whose formation is assumed in the protonation of nitrous acid, are computed as follows (energies are in millihartree~):~'AaE(+) = -10.08, AE(2df) = -94.62, AE(QC1SD) = +13.19. From these computed corrections, a -205.73223 au value is obtained for the Gaussian-1 absolute energy of the nitrosohydronium ion. From the Gaussian-1 total energy of trans-nitrous acid (see Table VI) and the zero-point energies of the HN02 and H2N02+species (see Tables IV and VI), a PA of 187 f 2 kcal mol-', a t 298 K, is obtained for the nitrous acid. From the experimental heats of formation of gaseous H N 0 2 and H+,a theoretical heat of formation of 160 f 2 kcal mol-' is obtained for the nitrosohydronium ion la.'3b It is interesting to point out that this value is in very good agreement with the experimentally measured heat of formation of gaseous H2N02+ from the hydration of Such agreement suggests that the nitrosohydronium ion which has been structurally characterized by MIKE and CAD spectrometry as the only observed isomer of H2N02+and whose stability has theoretically been evaluated in the present study corresponds to the H20-NOt species whose formation and stability have been previously reported by Kebarle and co-workers. Acknowledgment. We express our gratitude to Prof. F. Cacace for helpful discussions and encouragement, and to the Minister0 dell'Universid e della Ricerca Scientifica e Tecnologica (MURST) and the Consiglio Nazionale delle Ricerche (CNR) of Italy for financial support. The Italian NIC project is also gratefully acknowledged for a generous gift of IBM-3090-600J computational time. Registry NO. HNOI, 7697-37-2; HZN02+, 36180-50-4; NO, 1445293-8; NOz, 14797-65-0; C z H 5 0 N 0 , 79-24-3; C H 3 0 H , 67-56-1; CH30NO, 624-91-9; CH,NOzHt, 80729-10-8. (31) The absolute energies at the various employed levels of theory for the cis and trans isomers of nitrous acid are as follows (in atomic units): (i) cis isomer, -205.32120 (MP4/6-31 I+G**), -205.41878 (MP4/6-31 IG**(Zdf)), -205.303 39 (QClSD(T)/6-31 IG**); (ii) trans isomer, -205.322 89 (MP4/ 6-31 l+G**), -205.41724 (MP4/6-311G**(Zdf)), -205.30291 (QCISD(T)/6-31 lG**). (32) The abeolute energies at the various employed levels of theory for the nitrosohydronium ion (la) are as follows (in atomic units): -205.65080 (MP4/6-31 l+G**), -205.735 84 (MP4/6-311G**(2df)), -205.627 53 (QCISD(T)/6-31 IG**).
