The structure and ground state dynamics of Ar-IH

JOURNAL OF CHEMICAL PHYSICS VOLUME 111, NUMBER 13 1 OCTOBER 1999

The structure and ground state dynamics of Ar–IHA. McIntosh, Z. Wang, J. Castillo-Chara, R. R. Lucchese, and J. W. BevanChemistry Department, Texas A&M University, College Station, Texas 77843-3255

R. D. SuenramOptical Technology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

A. C. LegonSchool of Chemistry, University of Exeter, Stocker Road, Exeter, England EX4 4QD

~Received 20 May 1999; accepted 7 July 1999!

The structure and ground state dynamics of the atom–diatom dimer interaction between Ar and HIhas been investigated by microwave and near infrared supersonic jet spectroscopy.Ab initiomolecular orbital calculations were used to provide greater insight into the nature of the interaction.The ground state is shown to be in the isomeric form Ar–IH withRcm53.9975(1) Å, u5149.33(1)° for thenormal isotopomer andRcm53.9483(1) Å,u5157.11(1)° for Ar–ID. Thepotential surface from anab initio molecular orbital calculation was scaled and shifted to yield anonlinear least-squares fit of the rovibrational state energies to the experimental data. The groundstate potential energy surface obtained in this manner has a barrier between the Ar–IH and Ar–HIisomers of 88.5 cm21 with respect to the global minimum. Such calculations are also used to predictthe presence of localized states in the secondary minimum associated with isomers Ar–HI andAr–DI. Attempts to experimentally identify transitions associated with the latter were unsuccessful.The ground state, Ar–IH isomeric structure, contrasts with the corresponding ground state of theother members of the homologous series Ar–HX~X5F, Cl, and Br! in which the Ar is bound to theproton. © 1999 American Institute of Physics.@S0021-9606~99!00837-5#

HXp

avo

rafo

icsud

eeructe-eb

ng

rg

larenteg

ghharu

ofzle

hasam--mente.

a-tia-

or-i-

lyndvi-

iga-ing

aspec-T-inginu-he

INTRODUCTION

Investigations of the van der Waals complexes Rg–(X5F, Cl)1,2 using high-resolution gas phase spectroscohave been particularly fruitful. Indeed, these species hbecome prototypical systems for the investigationvibration–rotation-tunneling dynamics, the nature of vibtional predissociation and the development of modelscharacterization of the nature of intermolecular dynamSuch investigations are currently being expanded to inclmultibody interaction forces in clusters of the form3–7

RgnHX (X5F, Cl) based on the availability of the pairwisinteractions. The substitution of Cl for F in the van dWaals interactions of this type has been shown to proddramatic changes in the nature of the intermolecular potial energy surfaces and other properties.8 Consequently, substitution of Br or I in these species should produce furthdramatic changes. There have only been a restricted numof high-resolution spectroscopic studies of Ar–HBr usimicrowave9 and far infrared spectroscopy ofn2

1 fQ(J)branch10 and a recent characterization of the potential enesurface in then51 HBr stretching vibration of the dimer.11

A theoretical treatment of the ground intermolecupotential12 based on the previous microwave study had pdicted almost equal energy equilibrium isomers Ar–HBr aAr–BrH. The ground state of the complex was demonstrato be the Ar–HBr dimer with the isomer Ar–BrH bein10.88 cm21 above that ground state.

There have been no corresponding published hiresolution studies of the interaction between Ar and HI tpermit an unequivocal assignment of the ground state st

5760021-9606/99/111(13)/5764/7/$15.00

Downloaded 07 Jun 2002 to 128.104.69.200. Redistribution subject to A

yef-r.e

en-

rer

y

r-

dd

-tc-

ture of this dimer. There have been unpublished reportsattempts to observe the Ar–HI isomer using pulsed nozFT ~Fourier transform! microwave spectroscopy13 but theyproved unsuccessful. A different series of experimentsalso been completed on the photoinitiated chemical dynics in Ar–HBr and Ar–HI.14,15 These previous photodissociation studies of Ar–HI in a supersonic molecular beaexpansion did give proton distributions that were consistwith interpretation of dissociation of an Ar–IH ground statMore recently, there has been a REMPI~resonantly en-hanced multiphoton ionization! (211) investigation ofAr–HI.16 However, in this study, it was not possible to rottionally resolve the spectrum and thus permit a differention between a possible ground state having an Ar–HIAr–IH isomeric form. A simultaneously performed equilibrium ab initio calculation did indicate that the preferred equlibrium structure was ArIH. However, that investigation onprovided information about the equilibrium geometries adid not provide evidence as to the nature of the ground robrational state of the complex. Furthermore, these investtions did not provide definitive evidence as to the tunneldynamics of the complex.

In this work, we shall report an analysis of then1 nearinfrared spectrum of the complex Ar–IH. This analysis wthen used to locate the quadrupole resolved rotational strum of Ar–IH recorded using a pulsed nozzle Fmicrowave spectrometer. It is expected that a correspondanalysis of the deuterated isotopomer of Ar–ID will contavaluable information for characterization of the intermoleclar potential so that isotopomer was also investigated. T

4 © 1999 American Institute of Physics

IP license or copyright, see http://ojps.aip.org/jcpo/jcpcr.jsp

ranaerc-eto

ionaldceth

suicte

le-r.fr-i

tiv

gaerd

as

le.0lta

rm

regc-noin

yrr

all

red

lsoism

at-uc-

Br,heterw-oreed,yl-entin-

trib-

ngIDm-

an-d tom

dy-m-we

the

re

ogyrba-

byralnd

ionlarthe

the

is7f

ion

us-

the

5765J. Chem. Phys., Vol. 111, No. 13, 1 October 1999 Structure and dynamics of Ar–IH

former permits an unequivocal confirmation of the rovibtional assignment. The latter, in addition, permits determition of the ground state structure of the dimer as the isomform Ar–IH. Furthermore, this data will be used in conjuntion with the results ofab initio calculations to estimate thenergy of the ground state of the Ar–HI isomer relativethe global ground state. In particular, the present calculatincluded a determination of the full intermolecular potentimodified to obtain agreement with experimental data, ancalculation of the rovibrational dynamics on that surfaThese calculations thus represent a much more detailedoretical study of this system than previously reported.16 Fur-thermore, the determined semiempirical potential energyface obtained in this combined experimental and theoretstudy was used to predict several transitions involving inmolecular vibrations which are as yet unobserved.

EXPERIMENT

The supersonic jet spectra of then1 band of Ar–IH wasfirst recorded in the 4.48mm spectral region using a tunabdiode laser cw~continuous wave! supersonic slit jet spectrometer at Texas A&M University with an InSb detectoThis spectrometer was capable of operating as a highquency wavelength modulation spectrometer using lockdetection of the second derivative signal up to 1 MHz.17 Thespectrometer has been shown to have an ultimate sensiconsistent with a minimum absorbance of two parts in 107 orbetter. The supersonic jet expansion was formed from aeous reservoir consisting of typically 1% HI in Ar as carrigas and sustained at 2.03105 Pa. The expansion was formethrough a 12 cm long slit with 25mm width and all spectrarecorded with a single pass of the laser beam. The diode lwas a II–VI lead salt diode laser centered at 2230 cm21. Thetransition frequencies in the spectrum of the Ar–IH compwere calibrated to an estimated absolute accuracy of 0cm21 or better using standard frequencies of a simuneously recorded spectrum of N2O.18 Repeated efforts to ini-tially observe the spectrum of the alternate isomeric foAr–HI, proved unsuccessful.

The microwave spectra of both Ar–IH and Ar–ID werecorded in the frequency range 8 GHz–18 GHz usinnewly developed ‘‘mini’’ pulsed nozzle FT microwave spetrometer at the National Institute of Standards and Techogy at Gaithersburg, Maryland. This spectrometer, includits instrumental resolution~2 kHz! and its sensitivity, hasbeen described in detail elsewhere.19 Thus the following de-scription will focus on factors relevant to the current studThe supersonic pulsed nozzle expansions used in the cuexperiments were carried out with a 1 mmnozzle from bothstatic and flowing gaseous reservoirs consisting of typic1% HI in Ar as carrier gas and sustained at1.353105 Pa to2.703105 Pa. In certain cases 10% Ar and 1% HI wemixed in a He/Ne carrier particularly for attempted broaband experimental searches~;1 GHz–1.5 GHz! for theAr–HI isomeric form of the dimer. Such searches aproved unsuccessful and will be the subject of further dcussion below. Corresponding searches for the isotopoAr–DI were equally unsuccessful.


--ic

s,a.e-

r-alr-

e-n

ity

s-

er

x01-

,

a

l-g

.ent

y

-

-er

The hydrogen iodide used in both experiments was cegorized as 98% pure and obtained from Matheson, Gloester, Mass. The broadband FTIR~Fourier-transform infra-red! spectrum of the gas shows the presence of HCl, Hand H2O as impurities. In the infrared laser studies of tdimer, this gas was used without further purification, afremoval of the first cut of gas taken from the cylinder. Hoever, for the microwave studies, the impurities proved mtroublesome. In this case, not only was a first cut removbut the hydrogen iodide was also distilled from the gas cinder after the cylinder had been pumped at liquid nitrogtemperatures. Despite this procedure, residual water conued to provide some problems and microwave spectra atuted to (H2O!2

20,21 and H2O–HI22 dimers were both simul-taneously recorded with that Ar–IH. The correspondisample of deuterium iodide used in observation of the Ar–spectrum was obtained from Cambridge Isotopes, Cabridge, Mass. Prior to use, the sample of DI in the jet expsion was subjected to a similar procedure as that usepurify HI. It is pertinent to note that the maximum deuteriuenrichment observed in the dimer was;80% due to partialisotopic exchange in the gas lines.

THEORETICAL CALCULATIONS

We have also studied the Ar–HI complex usingab initiomethods so that we can probe the molecular tunnelingnamics of the dimer. To study this system, we have coputed the interaction energy on a grid of points wherehave kept the HI bond length fixed at its equilibrium value23

of r e51.609 16 Å. Nine values of the distance betweenAr and I atoms were considered,R53.5 Å, 3.75 Å, 4.0 Å,4.25 Å, 4.5 Å, 4.75 Å, 5.0 Å, 5.5 Å, and 6.0 Å, and thewere seven values of the angleu ~H–I–Ar! consideredu50 Å, 30 Å, 60 Å, 90 Å, 120 Å, 150 Å, and 180° leading ta total ofM563 points calculated. At each point, the enerwas computed using second order Møller–Plesset pertution theory ~MP2! using modified versions of the mediumsized polarized basis sets developed by Sadlej.24,25The basisset for Ar was obtained following the procedure describedSadlej24 using the Hartree–Fock wave function of the neutatom to determine the polarization function contractions astarting with the basis set McLean and Chandler.26 The basissets were modified by uncontracting the last polarizatfunction and adding a basis function of one higher angumomentum with the same exponents as were part offunction which was uncontracted. In the basis set of I,function that was uncontracted was the lastd type contractedGaussian so that twof functions were then added to the basset. Thus for H we addedd functions with exponents 0.102and 0.0324, for Ar we addedf functions with exponents o0.1454 and 0.0509, and for I we addedf functions with ex-ponents 0.091 353 and 0.033 851. The final contractscheme was [email protected]/3.3.2# for H, @13.10.4.2/7.5.3.2# forAr, and @19.15.12.8/11.9.7.5# for I. The interaction energieswere then corrected for the basis set superposition erroring the counterpoise method of Boys and Bernardi.27 Allelectronic structure calculations were performed usingGAUSSIAN electronic structure package.28


ctrum was.

5766 J. Chem. Phys., Vol. 111, No. 13, 1 October 1999 McIntosh et al.

FIG. 1. Pulsed nozzle FT microwave spectra of resolved quadrupole transitions ofJ57←6 transitions in~a! Ar–IH and~b! Ar–IH, and Ar–ID recorded withbroadband scans from 14.0 GHz to 15.0 GHz. The former spectrum was recorded with a supersonic jet expansion of Ar and HI. The latter sperecorded with an expansion of Ar and DI, the presence of Ar–IH in the spectrum reflects partial isotope exchange involving the labile DI isotope

ah

sofen

rm

-

n

-

thetial

er

le-ns.Hz.

ianrd

--

al

iza-ntsfit.e I.ID

eg-

lsoing

The computed interaction energies were then fit toanalytical form using the two-dimensional interpolation tecnique proposed recently by Ho and Rabitz.29 In this tech-nique, the points are fitted using a linear combination of aof fitting functions which are obtained from the formalismthe reproducing kernel of the Hilbert space. In the currstudy, we used the fitting functions

Qk~R,y!5q1n52,m56~R,Rk!q2

n52~y,yk!, ~1!

where the point (Rk ,yk) is the kth computed point,y is re-lated to the angleu ~H–I–Ar! throughy5(12cosu)/2, andwhere the two one-dimensional kernels are defined as29

q1n52,m56~R,R8!5

1

14R.

27H 127

9

R,

R.J ~2!

and

q2n52~y,y8!511y,y.12y,

2 y.H 121

3

y,

y.J . ~3!

Thus the final potential is represented by the functional fo

Vab initio~R,u!5 (k51

M

akQk~R,~12cosu!/2!, ~4!

where the expansion coefficientsak are determined by requiring theVab initio reproduce theM computed points. Therovibrational states were then computed using the variatiomethod described in detail previously.30–32

RESULTS

A segment of theJ57←6 rotational transition ofAr–IH is shown in Fig. 1~a!. The resolved quadrupole components have a characteristic group ofF11←F transitions


n-

et

t

al

and a more widely spaced group ofF←F transitions. Theassignment was based on a model calculation usingground state rotational constant determined from the inirovibrational analysis, an assumed structure for the dimand the available quadrupole constantx0 for monomeric HI.This structure was based on estimated changes in theRcm

distance,~an elongation of 0.4 Å!, and u ~increase of 6°!relative to previous studies of Ar–BrH.11,12 The assignmentwas rapidly confirmed by prediction of other quadruporesolved lines associated with other rotational transitioThe estimated accuracy of the measured transitions is 2 kThe spectral data was then fitted to a Hamiltonian

H5HR1HQ1HSR, ~5!

that is the standard expression for the rotational Hamiltonof an effective semirigid linear molecule. To the standaexpression for the matrix ofHQ in the coupled basis setF5I 1J, we added diagonal elements forxJJ(J11) wherexJ is a coefficient describing theJ dependence of the coupling constantx(I ). During the fit it was found that an additional term, xJJ

2(J11)2, though small empirically im-proved the standard deviation, so it was also added.HSR isthe spin-rotation Hamiltonian, of which only the diagonelements1

2Mbb@(F(F11)2I (I 11)2J(J11)# were used.The resulting constants were determined from a diagonaltion of the Hamiltonian matrix. The errors on the constareflect the uncertainty of two standard deviation in theThe determined molecular constants are given in TablCorresponding transitions for the deuterated isomer, Ar–were similarly identified based on a refined structure. A sment of theJ57←6 transition is shown in Fig. 1~b! with anapproximately 50% D labelled sample. This spectrum ashows quadrupole-resolved transitions of the correspond


ndTalhea

haineer

evonig

oin

te

-

ran-ex-la-

heab-deec-

edeanas

-

.e

ter-si-on

ec-

ly-

s o

ot

er in


Ar–IH isomer, which occurred in this frequency region ais present due to incomplete deuteration of the sample.corresponding molecular parameters of this isomer aregiven in Table I. For this fit the center frequencies of tresolved doublet transitions associated with deuterium qurupole splitting were used. An interesting observation is tthe B0 for Ar–IH is less than the corresponding valueAr–ID, which is attributed to a coupling interaction. Thquadrupole constants of the Ar–IH and Ar–ID were detmined to be 21114.40~2! MHz and 21409.40~4! MHz,where the uncertainties quoted represent two standard dtions. These values were used to determine the corresping angles of 149.33° and 157.11°, respectively, given in F2, respectively, using the familiar expressionxa

5x0^(cos2 u)&1/2 under the assumption that the presencethe Ar atom does not modify the charge distribution at Ithe equilibrium form of the complex. TheRcm distance wasthen determined by optimizing the structure to the demined rotational constants, also shown in Fig. 2.

A segment of theR(7) to R(11) branches of the rovibrationally resolved spectrum of thev1 in Ar–IH is shown in

TABLE I. The rotational constants obtained fron the microwave analysiAr–IH and Ar–ID.

Ar–IH Ar–ID

B ~MHz! 1034.0832~4! 1052.3598~10!D ~kHz! 10.596~8! 6.943~18!H ~Hz! 21.68~4! 22.81~10!xa(MHz) 21114.40~2! 21409.40~4!xJ(kHz) 80.8~10! 21.1~4!xJJ(Hz) 27~10!Mbb(kHz) 1.56~18! 1.7~2!s ~MHz! 0.0020 0.0015

FIG. 2. Ground state structures of Ar–IH and Ar–ID. TheRcm values arebased on rotational constants determined from analysis of the two ispomers.


heso

d-t

-

ia-d-.

f

r-

Fig. 3. The quadrupole structure associated with these tsitions has collapsed and is not resolved in the currentperiments. It is pertinent to note that this spectrum is retively weak and is detected at the sensitivity limits of tspectrometer. This is a consequence of the rovibrationalsorption cross section of HI being two orders of magnitusmaller than corresponding cross sections in HBr. This sptrum lacks aQ branch and is characteristic of theS←Sband. It is interesting to note that the band origin is rshifted by 1.5491~13! cm21, indicating that the excited statdissociation energy of this complex is slightly stronger ththat of the corresponding ground state. A similar effect hbeen observed for the Ar–79BrH and Ar–81BrH isomerswhere the corresponding red shifts were 1.461 11~30! cm21

and 1.461 74~29! cm21, respectively. In contrast, the corresponding Ar–HBr complexes were blue shifted 0.336 65~11!cm21 and 0.339 70~12! cm21 for the 79 and 81 isomersObservedP(J) and R(J) transitions were used to fit thexpression

n5n01@B8J8~J811!2DJ8J82~J811!2#

2@B9J9~J911!2DJ9J92~J911!2#. ~6!

The rotational assignment can be rapidly confirmed by demining lower state combination differences for these trantions and comparing with the corresponding values basedthe microwave analysis of Ar–IH reported above. The sptrum of the complex was recorded from;2226 cm21 to

TABLE II. The n1 rovibrational constants obtained from the infrared anasis of Ar–IH.

Constant Value (cm21)

n0 2228.032 57~4!B8 0.034 656 4~78!D8 2.96(32)31027

H8 22.1(34)310211

s 0.000 47

f

o-

FIG. 3. Segment of theR(7) to R(11) transitions ofn1 Ar–IH recordedusing high frequency wavelength modulation supersonic jet spectrometthe frequency range 2228.5 cm21–2228.95 cm21.


e

te-edbw

esomd

ao

.eo

he

no

HIs oftheent

led

of

at

at

ow-ve

teec

onc-

ilib-


2230 cm21 and transition frequencies associated withP(4)to P(19), R(3) to R(21) recorded. In the fittings of thestransitions to the energy expression given in Eq.~6! theground state is fixed to the previously and precisely demined values ofB0 andDJ

0 available from microwave spectroscopy. The resulting fit is given in Table II. The estimatuncertainties reflect two standard deviation from the fit. Asolute frequency accuracy of the measured transitions;0.001 cm21.

Using the ab initio calculations described above, whave predicted the rovibrational energy levels for both imeric systems, Ar–IH and Ar–ID, assuming that the safixed r potential energy surface. We have also consideremodified version of the potential of the form

Vscaled~R,u!5c1Vab initio~c2R1c3 ,u!, ~7!

where the three parameters,c1 , c2 , and c3 were varied toobtain the best fit of the spectroscopic constant to the avable experimental data. This modified potential is a type‘‘morphed’’ potential as discussed by Meuwly and Hutson33

The spectroscopic constants computed with both of thpotentials are given in Table III. The rotational constantsthe HI molecule used in the kinetic energy term in tHamiltonian were those of the free diatomic, 6.341 965 cm21

for HI and 3.223 077 2 cm21 for DI. The uncertainties usedto obtain the fits given in Table III were based primarily oan estimate of the error in the predicted spectroscopic c

TABLE III. Predicted spectroscopic constants fromVab initio and Vscaled.Also given are the experimental data used in the fit with the estimauncertainities due to the use of the same potential for both isotopic spand the dimensionless standard deviations~DSD! of the two sets of pre-dicted constants from the experimental data.

Vab initio Vscaled Experiment Uncertainty

Ar–IH B0/1022 cm21 3.292 3.454 3.449 0.01Ar–IH DJ/1027 cm21 3.43 3.54 3.53 0.01Ar–IH ^P1(cosu)& 20.799 20.817Ar–IH ^P2(cosu)& 0.585 0.603 0.610 0.004Ar–IH Du/1026 245.8 240.6 244.3 1.5Ar–ID B0/1022 cm21 3.345 3.505 3.510 0.01Ar–ID DJ/1027 cm21 2.09 2.34 2.33 0.03Ar–ID ^P1(cosu)& 20.909 20.912Ar–ID ^P2(cosu)& 0.754 0.761 0.771 0.004Ar–ID Du/1026 211.3 211.2 215.5 1.5DSD 10.40 1.81


r-

-as

-ea

il-f

sef

n-

stants due to the use of the same potential for the DI andforms. The changes due to the shift in the center of masthe diatomic are included in the calculation, however,changes in the intermolecular potential due to the differvibrational state of the diatomic are not included.

The values of the parameters for the optimized scapotential werec151.04060.004,c250.98760.008, andc3

50.1460.03 Å. The potentialVscaledis given in Fig. 4. Herewe can see that the global minimum is at theu5180° Ar–IHgeometry and there is a local minimum at theu50° Ar–HIgeometry which has much lower curvature in the directionthe bending motion. The minima are atVscaled(R53.79 Å,u5180°)52220.0 cm21 and Vscaled(R54.49 Å,u50°)52172.7 cm21. There is also a secondary minimumVscaled(R54.18 Å,u544°)52170.5 cm21. The saddle pointbetween the Ar–IH and Ar–HI geometries occursVscaled(R54.22 Å,u598°)52131.5 cm21. In Table IV wehave given the spectroscopic constants for a number of llying states for this potential. All of the energies are relati

dies

FIG. 4. A contour plot of the ground state potential of Ar–IH as a functiof u andR. The plot indicates a global minimum consistent with the struture Ar–IH and a secondary minimum associated with the isomeric equrium form Ar–HI. The contour labels are in units of cm21.

tively.
TABLE IV. Predicted spectroscopic parameters for unobserved states from theVscaledpotential. The values ofn for the IH and ID isotopomers are relative to the energies of the Ar–IH and Ar–ID ground states, respec
n/cm21 B/1022 cm21 D/1027 cm21 ^P1(cosu)& ^P2(cosu)& q/1022 cm21

Ar–IH stretch 23.09 3.256 3.97 20.661 0.506Ar–IH P bend 42.67 2.964 2.78 20.270 0.212 20.410Ar–HI ground state 7.89 2.960 1.50 0.638 0.350Ar–HI stretch 29.85 2.913 1.15 0.575 0.320Ar–HI P bend 26.71 3.090 2.30 0.245 20.057 0.025Ar–ID stretch 25.18 3.322 3.07 20.848 0.653Ar–ID P bend 34.69 3.380 2.64 20.628 0.389 0.211Ar–DI ground state 15.19 2.915 2.23 0.772 0.461Ar–DI stretch 34.35 2.699 2329.73 0.649 0.341Ar–DI P bend 28.85 3.054 2.66 0.506 0.116 0.003


lu

ain

wo

isolatthtibedthadl-linci

thsrea

iaeI

r-

cro-er

tterAr–

n-

en-the

luesrH

-HI

hislo-

sil-

hislo-ialund

thatbe

es

d toto

he

-

n-ter-

forerteasl-foredthey tose ofnt by

ndwi


to the ground state energy that for this potential has a vaof 2146.43 cm21 for Ar–IH and 2158.87 cm21 for Ar–ID.The difference in the curvature in the bending motion calso be seen in the energy required for exciting the bendmode. In the Ar–IH minimum, this energy is 42.74 cm21

whereas in the Ar–HI minimum the energy is 18.89 cm21.The contour plot of the potential energy surface sho

in Fig. 4 can be contrasted with the corresponding plotsother members of the homologous series Ar–HX~X5F, Cl,and Br!. The differences can be rationalized in terms of dpersive forces being relatively greater than the dipinduced-dipole forces thus giving rise to the ground stAr–IH structure. In the case of the other members ofhomologous series the opposite occurs. Another interescharacteristic of the potential of the Ar–IH complex canseen in the influence of the electron distribution centerethe H atom being outside the effective Pauling radius ofhalogen. Its effect is reflected in the development of anditional minimum in the potential contour plot. Figure 5 ilustrates other aspects of the potential. The dashedshows the minimum energy path in the potential as a fution of each angle for fixed bending angle. The solid lineobtained by fixing the angle and calculating the energy ofground intermolecular stretching state. The energy levelthe bending states of the two isomers Ar–IH and Ar–HI ashown. We can see that the inclusion of the zero-pointergy of the intermolecular stretching mode has only a smeffect on the height of the barrier in the bending potentWe further note that both of theP bending states are abovthe potential energy barrier separating the isomers Ar–and Ar–HI.

CONCLUSIONS

We have recorded the infrared spectrum of then1 vibra-tion in Ar–IH in a supersonic slit jet expansion. This info

FIG. 5. Minimum energy path energy profile for the Ar–HI~described intext! illustrating a global minimum associated with the isomer Ar–IH athe prediction of two secondary minima with a bound state associatedthe isomeric form Ar–HI.


e

ng

nf

-eeeng

ate-

ne-

seofen-lll.

H

mation was used to predict and then to observe the miwave spectrum of the ground vibrational state of the isomusing a pulsed nozzle FT microwave spectrometer. The lastudies have been extended to include the isotopomer,ID. The rotational constants andxaa quadruple constantsdetermined for the Ar–IH and Ar–ID isotopomers are cosistent with Rcm53.9975(1) Å, u5149.33(1)°, and Rcm

53.9483(1) Å, u5157.11(1)°. The increase of the angleon deuteration is consistent with a change in zero pointergy corresponding reduction of the average amplitude ofproton displacement. It is interesting to note that these vacan be compared with the corresponding values of Ar–Band Ar–BrD, Rcm53.917(1) Å, u5143.72(1)° and Rcm

53.850(1) Å,u5150.11(1)°. Attempts to observe the transitions associated with alternative isomeric species Ar–and Ar–DI were not successful. The ground state of tdimer thus is in contrast to other members of the homogous series Ar–HX (X5F, Cl, Br) in which the ground statehave the isomeric form Ar–HX. A combination of the avaable experimental data withab initio calculations has beenused to model the rovibrational tunneling dynamics. Tmodeling yields a potential energy surface in which the gbal minimum has the structure Ar–IH. The model potentenergy surface predicts a secondary minimum with a bostate corresponding to the isomeric form Ar–HI that is;7.6cm21 less stable than the ground state. We emphasizethis latter value is an approximate value and needs torefined when further experimental information becomavailable.

The conclusions presented here can also be comparethe results of previous investigations. We are now ableconfirm the ground state Ar–IH structure inferred from tproton translational energy distributions in UV~ultraviolet!photolysis experiments using the high-n Rydberg time-of-flight technique by Wittig and co-workers.14,15 In additionour estimatedD05143 cm21 for the Ar–IH system is somewhat deeper than the value of 93 cm21 determined from thephotolysis experiments of Heaven and co-workers.16 Ourvalue forD0 would imply that the sequence ofD0 values forthe homologous series of complexes ArHX is strictly icreasing, where the dissociation energies previously demined areD05101 cm21 for ArHF,34 D05115 cm21 forArHCl,35,36 andD05121 cm21 for ArHBr.12,35

ACKNOWLEDGMENTS

We are grateful to the National Science Foundationsupporting this research. A.L.M. wishes to thank the RobA. Welch Foundation for financial support in form of thaward of predoctoral fellowship. The support of the TexA&M University Supercomputing Facility is also acknowedged. ACL thanks the EPSRC of the United Kingdomthe award of a Senior Fellowship. Dale Brugh is also thankfor his assistance in recording the spectra. Throughoutcourse of this work, certain name brands were necessaradequately describe the experimental parameters. The uthese names should not be construed as an endorsemeNIST in any way.

th


ev

em

ys

y

P.

ld,

an

d C

er

In-

ol.

hys.

D.

an,

em.

hem.


1K. R. Leopold, G. T. Fraser, S. E. Novick, and W. Klemperer, Chem. R94, 1807~1994!.

2cf. contributions reviewed in J. M. Hollas and D. Philips,Jet Spectroscopyand Molecular Dynamics~Blackie Academic and Professional, 1995!.

3T. Schroder, R. Schinke, S. Liu, Z. Bacic, and J. W. Moskowitz, J. ChPhys.103, 9228~1995!.

4A. McIlvoy, R. Lascola, C. M. Lovejoy, and D. J. Nesbitt, J. Chem. Ph95, 2636~1991!.

5A. McIlvoy and D. J. Nesbitt, J. Chem. Phys.97, 6044~1992!.6A. Ernesti and J. M. Hutson, Faraday Discuss.97, 119 ~1994!.7S. M. Cybulski, M. M. Szczesniak, and G. Chalasinski, J. Chem. Ph101, 10708~1994!.

8T. D. Klots and H. S. Gutowsky, J. Chem. Phys.91, 63 ~1989!.9M. R. Keenan, E. J. Campbell, T. J. Balle, L. W. Buxton, T. K. Minton,D. Soper, and W. H. Flygare, J. Chem. Phys.72, 3070~1980!.

10D. W. Firth, M. A. Dvorak, S. W. Reeve, R. S. Ford, and K. R. LeopoChem. Phys. Lett.168, 161 ~1990!.

11J. Han, A. L. McIntosh, C. L. Hartz, R. R. Lucchese, and J. W. BevChem. Phys. Lett.265, 209 ~1997!.

12J. M. Hutson, J. Chem. Phys.91, 4455~1989!.13A. C. Legon~unpublished!.14C. Jacques, L. Valachovic, S. Ionov, E. Bohmer, Y. Wen, J. Segall, an

Wittig, J. Chem. Soc., Faraday Trans.89, 1419~1993!.15J. Segall, T. Wen, R. Singer, C. Wittig, A. Garcia-Vela, and R. B. Gerb

Chem. Phys. Lett.207, 504 ~1993!.16T. Suzuki, H. Katayanagi, and M. C. Heaven, J. Phys. Chem.101, 6697

~1997!.17A. L. McIntosh, Z. Wang, R. R. Lucchese, and J. W. Bevan~in prepara-

tion!.


.

.

.

s.

,

.

,

18G. Guellachvilli and K. N. Rao,Handbook of Infrared Standards~Aca-demic, New York, 1986!.

19R. D. Suenram, J. U-. Grabow, A. Zuban, and I. Leonov, Rev. Sci.strum.70, 2127~1999!.

20E. Zwart, J. J. ter Meulen, W. L. Meerts, and L. H. Coudert, J. MSpectrosc.147, 27 ~1991!.

21L. H. Coudert, F. J. Lovas, R. D. Suenram, and J. Hougen, J. Chem. P87, 6290~1987!.

22A. L. McIntosh, T. Walther, R. R. Lucchese, and J. W. Bevan, R.Suenram, and A. C. Legon~unpublished!.

23K. P. Huber and G. Herzberg,Molecular Spectra and Molecular StructureIV. Constants of Diatomic Molecules~Van Nostrand Reinhold, New York,1979!.

24A. J. Sadlej, Theor. Chim. Acta79, 123 ~1991!.25A. J. Sadlej, Theor. Chim. Acta81, 339 ~1992!.26A. D. McLean and G. S. Chandler, J. Chem. Phys.72, 5639~1980!.27S. F. Boys and F. Bernardi, Mol. Phys.19, 553 ~1970!.28GAUSSIAN 94, M. J. Frischet al. ~Gaussian, Inc., Pittsburgh PA, 1995!.29T.-S. Ho and H. Rabitz, J. Chem. Phys.104, 2584~1996!.30J. Han, A. L. McIntosh, Z. Wang, R. R. Lucchese, and J. W. Bev

Chem. Phys. Lett.265, 209 ~1997!.31A. McIntosh, A. M. Gallegos, R. R. Lucchese, and J. W. Bevan, J. Ch

Phys.107, 8327~1997!.32A. Quinones, G. Bandarage, J. W. Bevan, and R. R. Lucchese, J. C

Phys.97, 2209~1992!.33K. Meuwly and J. M. Hutson, J. Chem. Phys.110, 8338~1999!.34J. M. Hutson, J. Chem. Phys.96, 6752~1992!.35J. M. Hutson, Annu. Rev. Phys. Chem.41, 123 ~1990!.36J. M. Hutson, J. Chem. Phys.89, 4550~1988!.


https://www.researchgate.net/publication/246355091_Handbook_of_Infrared_Standards_II?el=1_x_8&enrichId=rgreq-8a35e6936fae1a0acdf91896f9c3883c-XXX&enrichSource=Y292ZXJQYWdlOzIzNDg4ODE1NztBUzoxMDQ2NTYwOTcyNTEzMjhAMTQwMTk2MzM2NjA1MA==

https://www.researchgate.net/publication/246355091_Handbook_of_Infrared_Standards_II?el=1_x_8&enrichId=rgreq-8a35e6936fae1a0acdf91896f9c3883c-XXX&enrichSource=Y292ZXJQYWdlOzIzNDg4ODE1NztBUzoxMDQ2NTYwOTcyNTEzMjhAMTQwMTk2MzM2NjA1MA==

The structure and ground state dynamics of Ar-IH

Documents

Transcript of The structure and ground state dynamics of Ar-IH