J. Phys. Chem. 1995, 99, 11667-11672
11667
Experimental and Theoretical Characterization of the Gas-Phase NS02- Ion Nelson H. Morgon, Harrald V. Linnert, and JosC M. Riveros" Instituto de Quimica, Universidade de Siio Paulo, Caixa Postal 26077, Siio Paulo, Brazil, CEP 05599-970 Received: February 20, 1995; In Final Form: May 5, 1995@
The gas-phase iodmolecule reaction of N H 2 - with S02F2 proceeds rapidly to yield NS02- (85%) and HNSOzF(15%). The NS02-, a long-sought species, was studied in a number of gas-phase acid-base bracketing experiments by FT-ICR leading to a AHoacidof 1381 21 kJ/mol for HNS02 at a temperature of 333 K. A lower limit of 3.49 eV has been set for the electron affinity of NS02'. Ab initio calculations using basis sets corrected in the valence region by the generator coordinate method yield a m a c i d of 1386 kJ/mol for HNSO2 at 298 K while the isomeric HOS(0)N is predicted to have a A H o a c i d of 1281 kJ/mol. The electron affinity of the N S O i radical is calculated to be 3.76 eV. Comparison has also been made with the previously identified NSO- ion for which our calculations predict a proton affinity of 1452 kJ/mol which compares very favorably with the experimentally derived value of 1439 f 21 kJ/mol. The optimized molecular structures and harmonic vibrational frequencies obtained at the MP2 level provide a valuable guide toward distinguishing these species.
Gas-phase ion chemistry has played an important role over the years in unveiling the intrinsic chemical reactivity of simple molecules under solvent-free conditions. At the same time, considerable insight has been gained on the structure and stability of ionic species from two sources: (1) the continuous development and improvement of experimental techniques in mass spectrometry, ion cyclotron resonance, flowing afterglow, photoionization, and ion beams; (2) theoretical calculations of varying degrees of sophistication which have recently provided a lively interplay with experiments to an extent almost unparalleled in other branches of chemistry. The study of anions has been a particularly fruitful and challenging area of gas-phase ion chemistry despite the fact that few negative ions can be produced directly by conventional ionization or electron attachment processes. Yet, the use of carefully tailored iodmolecule reactions has considerably broadened the horizon of the synthesis of anions in the gas phase.' Likewise, the theoretical description of anions has witnessed substantial progress in ways of dealing with the diffuse nature of the electronic cloud for these systems.2 A particularly interesting situation arises for simple stable anions which can been characterized in the gas-phase but which are not readily isolated in condensed media. Over the years, species such as XeCl-,3 N b - $ H30-,5 NPF-,6 HSO-,' and FzAsS-,* to name a few, represent good examples. During the investigation of the mechanism of nucleophilic iodmolecule reactions in simple inorganic sulfur compound^,^ an easy way has been found to generate the sulfonyl imide ion, NS02-, a heretofore elusive anionic species isoelectronic with SO3. While the synthesis of N-sulfonylamines at low temperatures was reported in the late 1960s,I0these compounds were known to undergo facile reactions at room temperature'' or to yield cyclic species.I2 Despite its simplicity, the successful isolation of NSO2- had in fact not been reported until the synthesis of Cs+NS02- was announced in 1982.13 Identification was based on an analysis by field ionization mass spectrometry of the Cs+ salt derivatized to the corresponding P h P + salt.I4 Unfortunately, the X-ray structure could not be determined because of crystal disorder. Initial attempts to make Rh complexes with an NS02- ligand failed and lead to a complex @Abstractpublished in Advance ACS Abstrucrs, June 15, 1995.
0022-3654/95/2099-11667$09.00/0
bearing an NSO- ligand.I5 The difficulties encountered in isolating the NS02- anion were compounded when the evidence for the actual existence of Cs+NS02- was disputed by Chivers et aLI6 The combined analysis of the infrared, Raman, and NMR spectra of the compound purported to be CsfNS02- was shown to be consistent with an NSO- anion. Yet, more recent work has claimed the synthesis of NS02- complexes of Ru, Rh, Os, and Ir although the JR data must still be regarded inconclusive as to the actual nature of the ligand (NSO- or NSO2-).I7 In solution, both NS02- and its corresponding acid HNS02 have been proposed to be key intermediates in the hydrolysis mechanism of aryl sulfamates, although their transient existence could not be detected directly.18 We now report the ion chemistry leading to the formation of the NS02- ion and its thermochemical characterization based on gas-phase bracketing experiments. Ab initio calculations have also been carried out for the NS02- ion to provide structural information regarding these species and the preferred protonation site. The calculated proton affinity is in excellent agreement with the experimental value. Furthermore, calculations on the previously identified NSO- i ~ n , ' have ~ , ~ also been carried out in order to compare the theoretical proton affinity and to help in the controversy regarding the vibrational frequencies of NS02- and NSO-.
Experimental Section Experiments were performed on a homemade FT-ICR spectrometer interfaced to an IonSpec Fourier Transform Omega Data System which has been described previou~ly.'~ All studies were performed in a 1 in. cubic cell at a magnetic field of 1 T. The temperature of the cell in our experiments has been measured with a thermocouple and found to be in the range of 333 f 5 K, but more recent measurements using a Pt thermometer suggest somewhat higher temperatures. Amide ions (NH2-) were generated from gaseous NH3 (Matheson, electronic grade) at pressures of 8 x Torr with a 100 ms electron beam pulse of 6 eV energy. Typical trapping voltages for these experiments were in the range - 1.2 to - 1.4 V. A radiofrequency (rf)field of 6 MHz was applied to one of the trapping plates during 150 ms to remove low-energy electrons trapped in the ICR cell. Experiments were also carried 0 1995 American Chemical Society
Morgon et al.
11668 J. Phys. Chem., Vol. 99, No. 30, 1995 out with C2H5NH- ions for comparison. The latter ions were generated from the iordmolecule reaction of NH2- with C2H5NH2 (Matheson) in the spectrometer cell. The C2H5NH- ions were then isolated after 250 ms by ejection from the cell of all unwanted ions with short rf bursts at the appropriate frequencies. SO2F2 (Linde) was used at partial pressures ranging from 3 x lo-* to 5 x lo-* Torr. Particular care was taken to ensure that F-, F2'-, S02F-, and S02F2.- ions, formed by low-energy electron impact of sulfuryl fluoride were removed by ejection techniques. Electron ejection is particularly important in order to avoid having the latter two ions reappear at later reaction times. Likewise, SO2F3-, an iordmolecule product of sulfuryl fluoride20%2' was removed prior to the study of any further ion chemistry. Other reagents used in this work were obtained from commercial sources and subject to vacuum distillation, and freeze-pump-thaw cycles prior to introduction of the samples in the system. The inlet system was usually heated to temperatures around 50 "C. Attempts to establish the electron affinity of NSOi by photodetachment experiments of NS02- were carried out with a pulsed Nd:YAG laser/dye laser combination. In these experiments, the time base of the l T data system is extemally triggered by a pulse from the lamps of the laser running at 10 Hz while the Q-switch of the laser is gated at the appropriate time by the FT-ICR sequence through a Hewlett-Packard pulse generator.22
Computational Procedure The interest in the general structural features of the NS02system, its relative simplicity, and the uncertainties conceming the experimentally determined thermochemical parameters provided the major impetus for the theoretical description of the anion and the protonated forms at the ab initio level. Calculations were carried out with the GAUSSIAN/92 program23in the cluster of work stations available at the Computer Center of the Unversity of Campinas (CENAPAD-SP). The generator coordinate method (GCM)24was used, as described below, to improve the description of the diffuse character of the electron cloud of negative ions. This method provides an interesting altemative for these calculations while maintaining the size of the basis set amenable to the environment of work stations The basis sets used for the different atoms were obtained according to the following procedures: ( a ) Nitrogen. The DZVPD basis set25 for N was first decontracted, and the functions with repetitive exponents were eliminated, Le., one of the functions with a = 2.686 is excluded. This set was then used to perform a UHF calculation on N- in the 3P state and the weight functions24constructed and analyzed. Two s-type and two p-type diffuse primitive functions were added in the valence region to improve the diffuse nature of the electron cloud. The exponents of these functions obey the relationship24c
ai+l= ai2/ai-, where i represents the most external function. The purpose of these functions is to correct the asymptotic character of the weight functions and to describe the anion correctly in this region. Figure 1 shows the significant improvement obtained for the weight functions of the extemal orbitals in the valence region after addition of the diffuse functions. Additional functions for the core electrons do not contribute significantly to the weight functions of the intemal orbitals implying that
( 8 ) 1s
0~
0.93
I
o without mrr&lon 0 wllh correction
i l h ocorrection ~l
n with CorreCtion 0.40
P
P
0.00
0.15
- O " l :
,
P O
,
~
o wllhoul COneClion
o,Bo
, , ,
,(d) 2P, (W,
,
0 withoul correction
0 with COrreCtim
0 with correction
/
0.17
0.03 -5.50
,:~Pw,
,
'
' -2.17
'
'
1.17
Ma)
'
'
4.50
Wa)
Figure 1. Weight functions for the atomic orbitals (a) Is, (b) 2s, (c) 2p1,(d) 2pz for N- considering the DZVPD basis set without correction (continuous line) and with corrections for the s and p functions (dashed lines). Notice that in (a) and (b) the two basis sets yield the same results.
these electrons are well represented by the original functions. Since the objective of this analysis is to improve the representation in the valence region without unduly increasing the size of the basis set, the intemal s-type functions were then contracted again (the most intemal seven primitives in the original set) with addition of the eliminated primitive function (a = 2.686). (b) Oxygen and Hydrogen. An analogous procedure was used to construct the basis set for 0 and H. In the latter case, only two diffuse s functions were added for the valence region. (c) Sulfur. The same basic procedure used for N was adopted for S . The core electrons were substituted by the effective core potential (ECP) of Hay and Wadt.26 By comparing the behavior of the weight functions for the 3s orbital of S for the two cases: (i) with all electrons included; (ii) with the pseudopotential, the cutoff point of the basis set is chosen where the contribution of the basis functions to the core electrons becomes negligible with the use of the pseudopotential. This means that the core electrons are well represented by the pseudopotential and that functions with exponents larger than the cutoff point can be excluded. The behavior of the weight functions for the 3s and 3p orbitals with and without the pseudopotential are shown in Figure 2. These graphs reveal that both the pseudopotential and the valence region are well represented since they display the necessary behavior for the adequate description of the weight functions as described previ~usly.~'Corrections for the d orbitals were found not to be necessary. The final basis set used in the calculations can thus be summarized: (a) for N and 0 the original DZVPD basis set augmented by two type s and two type p diffuse functions and the intemal functions contracted to (71 1111/111111Ul). For the H atom, the orignal DZVP set with two diffuse s functions which after contraction of the three internal functions results in a (3 111/1) set. For S , two s and two p diffuse functions were added to the DZVPD set which with the use of the pseudopo-
J. Phys. Chem., Vol. 99,No. 30, I995 11669
The Gas-Phase NS02- Ion
the nuclei.28 It is known that the forces acting at the nuclei of a molecule at the equilibrium configuration should be rigorously zero according to the electrostatic Hellmann-Feynman theorem. While this is rarely the case even for the most elaborate theoretical calculation, for the systems calculated in this paper, the electric field at the nuclei never exceeded 0.03 au.
Results and Discussion
(A) Gas-Phase Ion Chemistry. The reaction of N H 2 - with S02F2 proceeds rapidly through the two channels shown in eq 2 with the respective branching ratios. The overall rate constant NH2-
+
SO2F2
/"
HF
'
+
2HF
1
..........
v .1
......,.............
...........
I 9%0
3.03
-1.63
7.70
Wa)
Figure 2. Weight functions for the atomic orbitals (a) 3s, and (b) 3p of the sulfur atom obtained with the DZVPD basis set modified for systems with all electrons (continuous lines) and with the pseudopotential (dashed lines). The cutoff point is represented by the vertical dotted lines. TABLE 1: Exponents of the Diffuse Functions Added To Improve the Behavior of the Weight Functions atom H N
a,+l
a,+2
S
0.040 69
S
0.064 99 0.051 48 0.086 19 0.063 68 0.054 34 0.043 41
0.010 15 0.019 81 0.016 02 0.026 10 0.018 98 0.019 44 0.012 54
P 0
S
P S
S
P tential results in the set (1 1111/11111/1) ECP. The exponents of the functions added according to eq 1 are listed in Table 1 for each atom. One way to test the adequacy of the basis set obtained by the generator coordinate method is to perform a calculation on simple systems (for example, in the corresponding diatomic) of molecular properties sensitive to the quality of the basis set, and to verify the correct description of the weight functions of some of the selected molecular orbitals. The calculated geometry, total energy, zero-point energy, vibrational frequencies, electron affinity, and electric field at the nucleus for N2 and N2- were compared with experimental values and with values obtained from theoretical calculations using extended basis functions. Our basis set was also tested with regard to compliance with mathematical properties.28 The calculated values compare well with previous MRCI calculations employing a 13s8p3d2flg basis set contracted to [654321] for N2,29 and differ by 2% in the bond length and by 0.04% in the total energy. Our calculated electron affinity for N2 at the MP2 level is -0.699 eV. A test of the quality of the basis functions used in the present calculations was also verified by the calculated electric field at
+
HNS02F-
0.15 (2)
+
NSO2-
0.85
cannot be measured accurately because of the competing reaction of NHz- with the background water present in the instrument. Yet, this rate constant is comparable to that of OHwith S02F2, determined to be close to the ADO collision limit? Ion isolation experiments for either product ion, HNSOzFand NSOZ-, reveal that both ions are unreactive toward the parent SOzF;?. The first observation is particularly interesting since it shows that no gas-phase oligomerization can take place on S by sucessive nucleophilic addition followed by HF elimination. The reaction of EtNH- with S02F;?yields exclusively EtNSOzF-. (B) Proton Affinity of NSOz-. The observation of reaction 2 indicates that both HNS02F- and NS02- are the conjugate bases of gas-phase acids stronger than HF. The fact that the most favorable reaction proceeds by elimination of two HF molecules is indicative of the high stability of the NS02- ion and the driving force provided by the formation of HF. This gas-phase iodmolecule reaction is in contrast with the neutral reaction of NH3 with SOzF;?which leads to the formation of ( N H Z ) ~ S O ; ?A. ~similar ~ kind of situation, Le., elimination of two HF molecules, had been previously observed in the gasphase reaction of NH2- with PF3 to yield the very stable NPFion.6 These results suggest that a high gas-phase acidity is to be expected for HNS02, and that protonation on nitrogen would be predicted as the most favorable form. Unfortunately, the proton affinity of the NS02- species cannot be directly measured from the usual gas-phase acid-base equilibrium experiments since the conjugate acid, HNSO;?, is not known as a stable species. Thus, an estimate of the proton afinity of NSOz-, or the gas-phase acidity of HNS02, was obtained by bracketing techniques. For these experiments, NS02- ions were first isolated in the cell after all the NH2- ions had reacted (typically 200-250 ms) and were then allowed to react with volatile compounds of known gas-phase acidities3' to establish the occurrence and the ease of proton abstraction in reaction 3.
+ HA -HNSO, + A?
NS0,-
(3)
Reactions were followed up to 5 s with HA pressures ranging from 5 x to 2 x lo-' Torr. No proton abstraction was observed for HCOOH ( m a c i d = 1443 f 12 kJ/mol), CH2(COMe);?( m a a c i d = 1438 f 8 kJ/mo1)?2 or MeCHClCOOH ( m a c i d = 1407 f 10 Id/mol) whereas rapid and complete proton transfer was observed with CF3COOH (LW'acid = 1351 f 17 kJ/mol). Experiments with nonvolatile acids of intermediate gas-phase acidities could not be studied since our spectrometer is not properly equipped for solid sample introduction. Furthermore, no attempt was made to study proton abstraction
Morgon et al.
11670 J. Phys. Chem., Vol. 99, No. 30, 1995
TABLE 3: Population Analysis Obtained at the MP2 Level for the Atomic Nuclei of NSOz', NSOz-, and the Protonated Forms molecule S 01 0 2 N H Is
NS02'" NS02HNS02 HOS(0)N
Ib
0.952 0.761 0.733 0.852
-0.274 -0.511 -0.263 -0.364
-0.274 -0.511 -0.288 -0.389
-0.403 -0.739 -0.428 -0.422
0.247 0.323
a Spin densities for NSO2' are S (0.189), 0 (0.148), and N (0.515) at the MP2 level.
of translation, vibration and rotation at this t e m p e r a t ~ r e .The ~~ gas-phase acidities obtained at the MP4(SDTQ)//MP2 level by this procedure lead to Id
IC
Figure 3. Optimized molecular geometries obtained at the MP2 level with the modified DZVPD basis for NS02' (Ia), NS02- (Ib), HNSOz (IC),and HOS(0)N (Id). (Bond lengths in angstroms and angles in degrees.) TABLE 2: Calculated Electronic Energies (in au) and Zero-Point Energies (in kJ/mol) for NSOz', NSOz-, and the Protonated Forms molecule" HF MP2 MP4 ZPE ~~
NS02' NS02HNSO? HOS(0)N
-213.930 -214.007 -214.553 -214.503
287 487 655 152
-214.615 -214.750 -215.290 -215.245
843 951 007 899
-214.654 -214.790 -215.331 -215.291
659 988 789 289
34 29 61 59
a For NSOi, the HF and MP2 calculations are obtained at the PUHF and PMP-2 level, respectively. The MP4(SDQT) energies were obtained with the MP2 optimized geometry.
from HCl (hEfacld = 1395 f 1 kJ/mol) due to the fact that the NH3, used to generate the reactant NH2- ions, is flowed together with the reagent gas. Our measurements therefore allow us to estimate
AHoaC,,(298K) for 02SN-H = 1386 kJ/mol AHoaCid(298 K) for NS(0)O-H = 1281 kJ/mol The first value is in good agreement with the experimental results even though the large uncertainty of our experimental value precludes a more detailed comparison. Likewise, the electron affinity of the NSOi radical was calculated from the MP4 energies corrected for the zero point energies: EA(NS0,') = 3.76 eV Such a high value is consistent with the lower limit established experimentally and for such a simple system is comparable to the electron affinity of the 'CN radical measured to be 3.86 eV.34 The theoretical calculations also yield a prediction for the other thermochemically important parameter, the bond dissociation energy: D,(O,SN-H)
An alternative approach to determine the stability of the NS02- species was to measure the electron affinity of NSO2' by photodetachment techniques. However, the wavelength threshold for NS02- disappearance could not be established since it is outside the range of the dye laser. Nevertheless, these experiments clearly demonstrated the ability to photodetach the electron with the fourth harmonic of the Nd:YAG laser at 266 nm but not with the third harmonic at 355 nm. Thus
EA(NS0,') > 3.49 eV
(C) Reactivity of NSOz-. An initial investigation of the S02N- as a nucleophilic reagent in gas-phase S N reactions ~ revealed that it is unable to promote displacement reactions with the methyl halides. @) Electronic Energies and Charge Distribution of NS02', NSOz-, HNSOz, and HOS(0)N. The molecular geometries of the different species were optimized at the MP2 level using CZ,,symmetry for NS02- and NSOi and C, for HNS02 and HOS(0)N. The calculated molecular structures are shown in Figure 3, and the implications of some of the molecular parameters are discussed in part E. Table 2 lists the electronic energies of all the species calculated at different levels of electron correlation, as well as the corresponding zero-point energies. The calculations clearly establish that nitrogen protonation leads to the more stable isomer. The proton affinities of NS02- for protonation at nitrogen and at oxygen were then obtained at 298 K by correcting the appropriate electronic energies for the contribution
= 438 kJ/mol
This value is comparable to the bond dissociation energy determined experimentally for NH3,35Do(H2N-H) = 446.4 f 1.2 kJ/mol. The Mulliken population analysis for NS02-, NSOi, HNS02, and HOS(0)N is presented in Table 3. This analysis clearly shows the anionic character of the N atom in NS02- and the importance in the addition of diffuse functions for N in order to represent this system adequately. Furthermore, this charge distribution clearly suggests the preference for nitrogen protonation to yield HNS02 which is calculated to be some 105 kJ/ mol more stable than the corresponding HOS(0)N isomer. (E) Molecular Structure and Vibrational Frequencies of NSOz-, NSOz', HNSOz, and HOS(0)N. The molecular structures obtained by optimizing the geometry at the MP2 level reveal some interesting details and allow for comparison with the bonding in molecules containing the SO2 The calculated S-0 bond lengths in the NS02' radical are almost identical to the 1.43 8, encountered in S0337 and other simple SO2-containing molecules,38and the overall geometry is very similar to that of so3. On the other hand, the NS02- is calculated to have a much longer S - 0 bond and a substantially smaller 0-S-0 angle as a result of the added electron at N. The calculated S-0 bond lengths are somewhat similar to those calculated by several authors for the X3CSO3- anions.39 In the meantime, the calculated S-N bonds for all the species reflect the double-bond character in these systems and no simple systems are available for comparison. Furthermore, the calculated geometric parameters for NS02- are particularly revealing when compared with the polymeric units (NS02)33-, which have
J. Phys. Chem., Vol. 99, No. 30,1995 11671
The Gas-Phase NSOz- Ion
TABLE 4: Vibrational Frequencies (in em-') and Intensities (in ludmol) Calculated for NS02symmetry v (cm-I). assignment intensity a] 1188 v l ,sym str 244 b2 1049 v4, asym str 35 1 a1
884 473 419 383
a1 b2 bi
27 32 20 27
v2
v3, sym bend v5, asym bend
v6. out-of-planebend Vibrational frequencies obtained at the MP2 level and scaled by a factor of 0.90. a
1.491
1.414
I.
Ib
TABLE 5: Calculated Electronic Energies (in au) and Zero-Point Energies (in kcaVmol) for NSO', NSO-, and the Protonated Forms molecule" HF MP2 MP4 ZPE NSO' NSOHNSO NSOH
-139.103 -139.152 -139.717 -139.696
768 202 069 725
-139.555 076 -139.691 887 -140.256 419 -140.220976
-139.600 533 -139.730471 -140.295 982 -140.263 598
17 17 47 46
For NSOI', the HF and MP2 calculations are obtained at the PUHF and PMP-2 level respectively. The MP4(SDQT) energies were obtained with the MP2 optimized geometry.
TABLE 6: Population Analysis Obtained at the MP2 Level for the Atomic Nuclei of NSO', NSO-, and the Protonated Forms mo1ecu 1e S 0 N H NSO'" NSOHNSO NSOH
0.939 0.137 0.610 0.728
-0.533 -0.336 -0.409 -0.409
0.406 -0.801 -0.447 -0.628
0.245 0.309
a Spin densities for NSO' are S (0.256), 0 (0.143), and N (0.601) at the MP2 level.
Is42
1.6%
IC
Id
Figure 4. Optimized molecular geometries obtained at the MP2 level with the modified DZVPD basis set for NSO' (Ia), NSO- (Ib), HNSO (IC), and HOSN (Id). (Bond lengths in angles and angles in degrees.) been determined to have S-0 bond lengths characteristic of SO2 groups but long S-N bonds resembling single bond values.40 Table 4 contains the predicted vibrational frequencies and intensities for the NS02- species obtained from the present calculation^.^^ The vibrational frequencies have been scaled by a factor of 0.90 following recent suggestions regarding the scaling of frequencies derived from ab initio calculation^.^^ Yet the actual accuracy of these predictions is still a critical question as empirical scaling is often necessary to obtain agreement with experiment as shown recently for systems such as SOF2, POFzand C ~ O F Z + .A~ ~tentative assignment of the calculated frequencies can be proposed by considering the vibrational modes of so3 reduced to CzVsymmetry.& An inversion of the relative order of the symmetric and asymmetric NS02- stretches is predicted in contrast with the usual trend observed even for the similar FS02- ion.45 On the other hand, the vibrational frequencies calculated for NS02- are significantly different than those of the isoelectronic so3 (Vsym = 1068 cm-I and Yasym = 1391 cm-1)?6 which may not be surprising in view of the calculated bond lengths and angles. (F) Theoretical Calculations for NSO', NSO-, HNSO, and HOSN. The fact that serious doubts have been raised about how to distinguish NS02- and NSO- by spectroscopic meansI6 and the fact that the proton affinity of NSO- has been previously estimated from bracketing experiment^^^ motivated the extension of our calculations to this ion and its protonated forms. The molecular geometries of the different species were optimized as before at the MP2 level and are shown in Figure 4. The NSO- also differs significantly from its isoelectronic counterpart SO2, and the lengthening of the S-0 bond is particularly noticeable. Similar comments to those advanced above can be made about the calculated molecular structures of the different species. Table 5 lists the electronic energies and the zero-point energies for each species obtained in a fashion similar to that used in part B. From these calculations, the acidity for nitrogen and oxygen protonation was determined at 298 K by correcting
TABLE 7: Vibrational Frequencies (in cm-') and Intensities (in W m o l ) Calculated for NSO- and Comparison with Experimental Frequencies sym v (cm-ly assignment intensity experimental freq (cm-I) a' 1204 asym stretch 355 1261: 1240' a' 948 sym stretch 207 991: 1040' a' 423 bending 32 4936 a Vibrational frequencies obtained at the MP2 level and scaled by a factor of 0.90. Reference 16. Reference 17. the electronic energies for the contribution from translation, vibrations, and rotation at this temperature. These corrections lead to the following gas-phase acidities at the MP4(SDTQ)// MP2 level: AHOaCi,(298K) for OSN-H = 1452 kJ/mol AHoa,,(298 K) for NSO-H = 1369 kJ/mol These values clearly argue for the preference of nitrogen protonation and the calculated acidity is in excellent agreement with the experimental value of 1439 f 21 k J / m ~ l . ~ ~ The electron affinity of the N O S radical is calculated to be
EA(0SN') = 3.53 eV while the corresponding bond energy is calculated to be D,(OSN-H)
= 482 kJ/mol
a comparatively high value for N-H bonds which finds comparable values only in those estimated for HNCO and HNcs.31 An analysis of the Mulliken population for NSO-, NSO', HNSO, and HOSN is listed in Table 6. By analogy with the arguments presented for Table 3, the charge distribution clearly suggests the preference for nitrogen protonation to yield HNSO, which is predicted to be 20 keal/mol more stable than the corresponding HOSN. Table 7 lists the calculated vibrational frequencies and intensities for NSO- and the assignment in this case is straightforward. These calculated frequencies are compared with the experimental frequencies obtained by Chivers and coworkers.I6 Good agreement is obtained between the calculated and experimental frequencies of ref 16 if an empirical scaling
11672 J. Phys. Chem., Vol. 99, No. 30, 1995 factor of 0.94 is used for the stretching vibrations and 1.04 for the bending vibration. This is in fact very analogous to what has been observed in recent calculations for systems like SOF2 and POF2- 43 and may have to be applied to the predicted frequencies for NS02-.
Conclusions The combination of experimental measurements and theoretical calculations presented in this paper clearly identify some of the main features of the long sought NS02- species. While the experimental gas-phase acidity experiments provide only an approximate value, the ab initio calculations firmly support this value. The fact that the calculations on NSO- are also in good agreement with previous experimental results strongly suggests that the theoretical approach used in these systems is very useful. In this respect, the generator coordinate method holds great promise as a tool for the adequate description of the electronic cloud for these systems, maintaining a moderate size basis set while providing valuable insight to experimental measurements. The molecular structures and vibrational frequencies derived for the NS02- and NSO- show some interesting features which justify further exploration. The vibrational frequencies derived from the theoretical calculations reveal that it should be possible to differentiate between NS02- and NSO- as a ligand by IR spectra, in particular in the region of the symmetric and antsymmetric stretches. Finally, the thermochemical values which have been obtained for the species of interest in our case provide a valuable contribution to the general field of gas-phase ion chemistry.
Acknowledgment. The authors wish to thank the SI0 Paul0 Science Foundation (FAPESP) for continuous grant support and the Brazilian Research Council (CNPq) for their support through their Young Investigator Fellowship (NHM) and Senior Research Fellowship (JMR) programs. We also wish to thank the Computer Center (CENAPAD-SP) and the Institute of Chemistry of the University of Campinas for a generous allotment of computing time. References and Notes (1) See, for example: DePuy, C. H.; Grabowski, J. J.; Bierbaum, V. M. Science 1982, 218, 955. (2) See, for example: Smith, B. J.; Radom, L. J. Phys. Chem. 1991, 95, 10549. (3) Riveros, J. M.; Tiedemann, P. W.; Breda, A. C. Chem. Phys. Lett. 1973, 20, 345. (4) Kleingeld, J. C.; Ingemann, S.; Jalonen, J. E.; Nibbering, N. M. M. J. Am. Chem. SOC.1983, 105, 2474. ( 5 ) Kleinceld. J. C.: Nibbering. N. M. M. Int. J. Mass Soectrom. Ion Phys. 1983, 45, 31 1. (6) Sullivan. S. A.; Beauchamp, J. L. Inora. Chem. 1978, 17, 1589. (7) (a) DePuy, C. H.; Bierbaum, V. M. TeTrahedron Lett. 1981, 22, 5129. (b) Bierbaum, V. M.; Grabowski, J. J.; DePuy, C. H. J. Phys. Chem. 1984, 88, 1389. (c) O’Hair, R. A. J.; DePuy, C. H.; Bierbaum, V. M. J. Phys. Chem. 1993, 97, 7955. (8) Larson, J. W.; McMahon, T. B. Inorg. Chem. 1987, 26, 4019. (9) Linnert, H. V.; Souza, L. A. G.; Riveros, J. M., manuscript in preparation. (10) Atkins, G. M.; Burgess, E. M. J. Am. Chem. Soc. 1967, 89, 2502. (11) (a) Atkins, G. M.; Burgess, E. M. J. Am. Chem. SOC. 1968, 90, 4744. (b) Burgess, E. M.; Williams, W. M. J. Am. Chem. SOC.1972, 94, 4386. (12) Banister, A. J. Phosphorus Su&r 1978, 5, 147. (13) Roesky, E.; Schmieder, W. E.; Isenberg, W.; Bohler, D.; Sheldrick, G. M. Angew. Chem., Int. Ed. Engl. 1982, 21,153. I
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