Infrared spectra of clusters of protonated nitric acid and water exhibit a marked change with cluster size, indicating that an intracluster reaction occurs with sufficient solvation. In small clusters, H_2O binds to a nitronium ion core, but at a critical cluster size the NO^+_2 reacts. A lower bound of 174 kcal/mol is found for the proton affinity of HNO_3

Cao, Yibin

Choi, Jong-Ho

Haas, Bernd-Michael

Johnson, Matthew S.

Okumura, Mitchio

English

Caltech Authors - Main

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lntracluster rearrangement of protonated nitric acid: Infrared spectroscopic 
studies of H+(HN03)(H20)n 
Yibin Gao, Jong-Ho Choi, Bernd-Michael Haas, Matthew S. Johnson, 
and Mitchio Okumura 
A. A. Noyes Laboratory of Chemical Physics, a) California Institute of Technology, 
Pasadena, California 91125 
(Received 9 August 1993; accepted 21 September 1993) 
Infrared spectra of clusters of protonated nitric acid and water exhibit a marked change with 
cluster size, indicating that an intracluster reaction occurs with sufficient solvation. In small 
clusters, H20 binds to a nitronium ion core, but at a critical cluster size the NO{ reacts. A lower 
bound of 174 kcal!mol is found for the proton affinity of HN03 • 
INTRODUCTION 
The large differences between gas phase and solution 
phase basicities highlight the dramatic effects of solvation 
on ion chemistry. Strong aqueous acids such as HN03 can 
possess a proton affinity in the gas phase that is higher than 
that of H20. By clustering water molecules sequentially to 
a protonated nitric acid, the structure and properties of the 
ion will evolve towards the solvated form. Investigations of 
these hydrated cluster ions can provide insights into mi-
croscopic aspects of solvation. 
Protonation of nitric acid occurs in the liquid phase 
under nearly anhydrous conditions, e.g., in neat nitric acid 
and in concentrated sulfuric acid, by the reaction H+ 
+HN03 -.NOt +H20. 1 Upon addition of water, the ni-
tronium ion (NOt> rapidly undergoes the reverse reaction 
and is therefore not present in aqueous solution. 
Experimental2•3 and theoretical4•5 studies have con-
cluded that gas phase protonated nitric acid H2NOt pos-
sesses several isomers. The most stable form is a weakly 
bound complex of NOt and H20 and, as in nonaqueous 
liquids, the most basic site is on the OH group. Cacace 
eta/. 3 found evidence for a second isomer, (HOhNO+, 
which lies 10-20 kcal/mol higher and is formed by the 
protonation of a terminal oxygen atom of HN03 • Ab initio 
calculations5 by Lee and Rice confirmed the structures and 
relative stability of the two isomers, but the computed pro-
ton affinity for nitric acid is 182 ± 4 kcal!mol, in disagree-
ment with Cacace's ICR bracketing measurement of 
168 ± 3 kcal/mol and closer to an earlier measurement2 of 
17 6 ± 7 kcal/mol. These values lead to large differences in 
the binding energy of the NO{(H20) complex (5-19 
kcal/mol). 
The existence of hydrated H2NOt was inferred by 
Fehsenfeld et a!., who studied the ion chemistry of HN03 
in a flowing afterglow apparatus.2 They observed rapid 
proton transfer from H30+ to HN03 , and attributed the 
decay of H2NOt to its association with H20. How-
ever, they were unable to detect hydrated clusters of pro-
tonated nitric acid, and postulated a fast reaction 
H+ (HN03 ) (Hp) +HP-H30+ (H20) +HN03 that 
would destroy the association products and convert NOt 
to HN03 • In an ab initio calculation, Grandinetti et a/. 6 
predicted that the lowest energy structure of 
H+(HN03)(H20) is NOt(H20h. Kay eta/. formed the 
clusters H+(HN03)(HP)n (n=l-12) by ionization of 
neutral clusters in a beam 7 and attributed a minimum in 
the size distribution at n = 4 to the formation of solvated 
ion pairs in the neutral complexes, but made no conclu-
sions regarding the ionic clusters. 
We have investigated the clusters H+(HN03)(H20)n 
for n = 0 to 4 by infrared vibrational predissociation spec-
troscopy. In this Communication, we present evidence for 
solvent-induced changes in the structure of protonated ni-
tric acid. 
EXPERIMENT 
The apparatus has been described elsewhere, 8 and is 
briefly described here. UHP helium was seeded with 
HN03/H20 vapor by passage over the surface of concen-
trated (70%) nitric acid at 0 ·c. The HN03/H20/He mix-
ture at a total stagnation pressure of 1000 Torr was then 
pulsed into a small channel where a high voltage pulse was 
applied between two electrodes to initiate a glow discharge. 
The resulting ions underwent many thermal collisions as 
the gas flowed through the channel and expanded into the 
vacuum, promoting cooling and clustering. The ion chem-
istry in our source was similar to that of high pressure 
discharges, producing the most stable ions. The ions were 
mass analyzed by a time-of-flight (TOF) mass spectrome-
ter. A pulsed tunable infrared laser, a LiNb03 optical para-
metric oscillator9 with 1.5 em -t resolution, was timed to 
excite clusters of a selected mass. Upon photon absorption, 
clusters could vibrationally predissociate. The resulting 
fragment ions were mass analyzed by a reffectron mass 
spectrometer. Infrared spectra were obtained by measuring 
the photofragment intensity as a function of the laser wave-
length. 
We typically generated H+(HN03)(HP)n clusters 
with n ranging from 0 to 7, as well as H 30 + ( H20) n and 
small amounts of No+(H20)n. We obtained the vibra-
tional predissociation spectra of H+ ( HN03 )( H20) n clus-
ters, with n = 0 to 4, in the range 2650-3850 em -I. 
RESULTS 
The infrared spectrum of protonated nitric acid (n 
= 0) had two distinct bands, one centered at 3626 em- 1 
J. Chern. Phys. 99 (11 ), 1 December 1993 0021-9606/93/99(11 )/9307 /3/$6.00 @ 1993 American Institute of Physics 9307 
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9308 Letters to the Editor 
TABLE I. Vibrational predissociation band centers for H+(HN03)(Hp). in cm- 1• 
H 3o+ HN03, v1 H20, Vt H20, VJ 
H bonded NO-H Symmetric Antisymmetric 
H+(HN03) 
H+(HN03)(H20) 
H+(HN03)(HPh 
H+(HN03)(H20h ~ 2600, ~ 3000 3519,3550 
H+(HN03)(H20)4 ~2700,~3000 3523,3550 
H20 monomer 
HN03 monomer 3551 
and the other a doublet with maxima at 3708 and 3716 
cm- 1 (Table I). These were redshifted by less than 50 
em- 1 from the symmetric and antisymmetric stretching 
bands of the water monomer. The only predissociation pro-
cess observed was 
(1) 
The dissociation signal was very weak and exhibited a non-
linear dependence on laser intensity, indicating that the 
photodissociation arose from a multiphoton process. 
Clusters of protonated nitric acid solvated by one or 
two water molecules, H+(HN03)(H20)n (n=1,2), be-
haved like the unsolvated H 2NOt, with a single water 
molecule evaporating upon vibrational excitation: 
H+ (HN03) (HzO)n ..... H+ (HN03) (HzO)n-1 +H20. 
(2a) 
In both clusters, only two bands were observed in the OH 
stretch region, slightly redshifted from the water symmet-
ric and antisymmetric stretches, respectively (Table 1). 
The dissociation signal was much stronger than for n = 0 
and depended linearly on laser intensity. For n = 3, two 
small features appeared in the 3550 cm- 1 region. 
For clusters H+ (HN03)(H20)n with n>2, we ob-
served the onset of a new photodissociation channel, loss of 
a nitric acid molecule, 
in competition with the water loss channel (2a). Although 
evaporation of nitric acid was a minor channel for n = 2 
predissociation, it was a major channel for n = 3 and the 
dominant channel for n = 4. The ratio of the nitric acid 
channel (2b) yield to the water channel (2a) yield was 1:8 
for n=2, and 9:1 for n=4. In n=3, the yield depended on 
the band excited: it was 7:1 upon excitation in the 3550 
em - 1 region and 2:1 in the 3600-3800 em -I region. 
Three new bands were observed for the clusters 
H+(HN03)(H20>J and H+(HN03)(H20) 4 in addition 
to the two H 20 bands (Fig. 1). Two were close to 3550 
em -I, the OH stretching frequency of the HN03 mono-
mer. The third was a broad band at 2600-2700 em -I. A 
similar band at 2670 em -I was observed by Lee and co-
workers in their studies of H 30+(H20)n clusters and 
assigned to the hydrogen-bonded OH stretches of the 
H 3o+ ion. 10 
3626 3708,3716 
3635 3715 
3636 3716 
3644 3717 
3644 3717,3733 
3657 3756 
DISCUSSION 
We can derive qualitative structural information about 
these clusters from their infrared spectra and photodisso-
ciation behavior. Protonated nitric acid, H+ (HN03), has 
two redshifted H 20 stretch bands and dissociates directly 
into NO{ and H 20. Coupled with the absence of bands 
near 3550 em -I, we conclude that protonated nitric acid is 
a weakly bound complex NO{(H20), in agreement with 
earlier studies. The structure calculated by Lee and Rice5 is 
a highly asymmetric top for which the symmetric H 20 
stretching mode would be an a type band and the antisym-
metric mode a b type band. This is consistent with the 
observed rotational contours, a single maximum at 3626 
cm- 1 and two closely spaced maxima at 3712 cm- 1. 
The similarity in the spectra and the photoproducts of 
H+(HN03) and its hydrated clusters (n= 1,2) indicates 
that the smaller hydrates are also complexes of an NO{ 
ion core and HzO ligands (Fig. 2). With additional 
ligands, the binding energy should be lower and the H 20 
stretching modes less perturbed. This inference is con-
firmed by the stronger photodissociation signal and its lin-
ear power dependence, as well as the smaller redshift of the 
H 20 bands. 
There is a striking difference in the properties of the 
larger clusters (n=3,4). The appearance ofnitric-acidlike 
bands near 3550 em -I and the onset of dissociation releas-
ing HN03 indicate that neutral HN03 exists intact in the 
clusters. The ~ 2650 em- 1 band and the bands in the 
3600-3800 cm- 1 region are evidence of H 3o+ and H 20, 
J! 
35 
30 
25 
·c: 20 
::J 
.e 15 
<( 
10 
5 
0~--~--~--~~~~~~~~ 
2650 2850 3050 3250 3450 3650 
Wavenumber (cm-1) 
FIG. 1. Vibrational predissociation spectrum of H+(HN03)(H20)4. 
The ordinate is the H30+ (H20)j photofragment signal, with laser-
independent background subtracted and normalized for laser intensity. 
J. Chern. Phys., Vol. 99, No. 11, 1 December 1993 
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Letters to the Editor 9309 
n= 1: 
H 0 fi 
\ II / 0 ·······N*-------- 0 
I II \ 
H 0 H 
HzQ OH 
·-.. ./ 2 
--~~-
n =4: t 
0 
~'-..._H 
HNo;······ 
FIG. 2. The proposed structures for H+ (HN03)(H20) and 
H+(HN03)(HP)4 • 
respectively. We therefore conclude that the larger clusters 
are complexes of an H3o+ core surrounded by water 
ligands and a neutral HN03 . 
H20 is more strongly bound than HN03 due to its 
more favorable hydrogen bonding,2 suggesting that the first 
solvation shell in the n =4 cluster is occupied by water 
ligands with HN03 in the second shell, as depicted in Fig. 
2. In both n=3 and n=4 clusters, channel (2b) dominates 
because the HN03 ligand is more likely to predissociate. 
The wavelength dependence of the branching ratio in 
the n = 3 cluster is surprising. HN03 evaporation is ap-
proximately three times more likely upon excitation of the 
HN03 band relative to excitation of the H20 bands. This 
behavior suggests that the predissociation process is mode 
selective. However, it is more likely that two nearly isoen-
ergetic isomers are present, one with a hydronium ion core, 
H3o+ (H20)z(HN03), and the other with a nitronium 
ion, NOi (Hp) 4 • The latter would absorb only at 3600-
3800 em- 1 and would produce only H20 fragments. The 
H30+ (H20)z(HN03 ) isomer would absorb at 3550 cm- 1 
as well, but would dissociate primarily by loss of HN03 . A 
mixture of the two isomers would thus appear to have 
wavelength-dependent branching ratios. 
Our results provide a lower limit on the proton affinity 
of nitric acid. From the nonlinear power dependence of the 
predissociation, we infer that dissociation requires at least 
two photons, and that the binding energy ofNOtCH20) is 
> 10.6 kcal/mol. Using known thermodynamic quanti-
ties~ 1•12 and a revised llii1 for NOt, 13 we can compute 
t::..H 1 for H2NOj and hence the proton affinity of HN03 • 
We find a lower limit on the proton affinity of 174 kcal/ 
mol, which disagrees with the currently accepted value3 of 
168 kcal/mol, but agrees well with the earlier measurement 
of 176 kcal!mol by Fehsenfeld et a/. 2 and the ab initio 
value of 182 kcal!mol. 5 
We found no evidence for the formation of the higher 
energy isomer (HO)zNO+ or its hydrates in our source. 
While this ion is covalently bound and may not predisso-
ciate upon photon absorption, the hydrates should. 
M0ller-Plesset (MP2) calculations5 predict that the 
(scaled) NO-H stretching frequencies lie near 3450 cm- 1, 
but no such bands were observed in any clusters. 
In conclusion, we have observed the intracluster reac-
tion NOt +2HP-+HN03+H3o+ upon sufficient solva-
tion. The driving force for overcoming the intrinsic stabil-
ity of gas phase NOt is the solvent stabilization of Hp+. 
This stabilization effect has also been observed in other 
studies. 14•15 Our technique provides a direct spectroscopic 
probe of solvent-induced phenomena in clusters; other sys-
tems are currently under investigation. 
Note added in proof The binding energy of 
NOt (H20) was recently measured directly and found to 
be 14.8 kcal/mol. 16 
ACKNOWLEDGMENTS 
We acknowledge the support of a National Science 
Foundation P.Y.I. Award No. CHEM-8957243, and the 
Dreyfus, Irvine, and AT&T Foundations. We thank Dr. 
Timothy Lee for helpful discussions. 
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lntracluster rearrangement of protonated nitric acid: Infrared spectroscopic studies of H^+(HNO_3)(H_2O)_n

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lntracluster rearrangement of protonated nitric acid: Infrared spectroscopic studies of H^+(HNO_3)(H_2O)_n

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