Khriachtchev, Leonid

Lignell, Antti

Pettersson, Mika

Räsänen, Markku

Helsingin yliopiston digitaalinen arkisto

JOURNAL OF CHEMICAL PHYSICS VOLUME 117, NUMBER 3 15 JULY 2002COMMUNICATIONSLarge blueshift of the H–Kr stretching frequency of HKrCl uponcomplexation with N2Antti Lignell,a) Leonid Khriachtchev, Mika Pettersson, and Markku Ra¨sa¨nenLaboratory of Physical Chemistry, P.O. Box 55, FIN-00014 University of Helsinki, Finland~Received 30 April 2002; accepted 15 May 2002!Interaction between HKrCl and N2 is investigated in a low temperature Kr matrix and by using abinitio calculations. Two configurations of the HKrClflN2 complex are found both computationallyand experimentally. The complexes show large monomer-to-complex blueshifts ~1112.9 and 132.4cm21 experimentally! of the H–Kr stretching vibration mode. The large blueshifts are attributed tothe enhanced ~HKr!1Cl2 ion-pair character of complexed HKrCl resulting in stronger H–Kr covalentbonding. © 2002 American Institute of Physics. @DOI: 10.1063/1.1491403#Several years ago, it was found that solid rare gas ~Rg!matrices become reactive when they are doped with hydro-gen atoms ~H! and suitable electronegative fragments ~Y!.1Upon thermal mobilization of hydrogen atoms, chemicallybound HRgY molecules are formed.2 Vibrational propertiesof the HRgY molecules are sensitive to their local environ-ment as evidenced by the large site splittings of the H–Rgstretchings of HArF ~;50 cm21, 2.5%! and HKrF ~;25cm21, 1.2%!.3,4 The strong environmental effects are mostprobably connected with the large polarity of the HRgY mol-ecules as well as with their relatively weak bonding. TheHRgY compounds show (HRg)1Y2 ion-pair character, re-sulting in large computational dipole moments ~4.5–9.4 D,1 D’3.34310230 C m!,2 comparable with salt moleculessuch as NaCl ~9.00 D!.5Until now, the experimental investigations of HRgYmolecules have concentrated on the properties of monomersisolated in rare gas matrices. Computationally, complexes ofXeH2 with water have previously been studied.6 In thiswork, we investigate both computationally and experimen-tally interaction between HKrCl and N2 . An intriguing ques-tion is how much the intermolecular interaction can stabilizeor destabilize HRgY molecules, and to what extent the com-plexation influences the properties of these molecules.A large number of various N2 complexes of hydrogencontaining molecules have been studied. Typically, complex-ation with N2 induces a monomer-to-complex redshifts in thehydrogen stretching vibrational mode. As a representativeexample, the HClflN2 complex shows a complexation-induced redshift of 219.1 cm21 ~20.7%! in solid krypton.7According to our best knowledge, no blueshifts of the hydro-gen stretching modes have been reported upon complexationwith N2 . Recently, several examples of blueshifting hydro-gen bonds have been found between different complexpartners.8In the experiments, HCl ~99%, CIL!, N2 ~99.9999%,a!Electronic mail: lignell@csc.fi9610021-9606/2002/117(3)/961/4/$19.00Downloaded 21 Sep 2007 to 128.214.3.62. Redistribution subject toAga! and Kr ~99.95%, Air Liquide! gases were mixed invarious proportions in a glass bulb. The matrix ratios usedwere HCl:N2 :Kr51:(0 – 10):1000. The samples were de-posited onto a CsI substrate kept at 27 K in a closed-cyclehelium cryostat ~APD, DE 202A!. The typical matrix thick-ness was 100–200 mm. The spectra were measured at 7.5 Kby a Nicolet 60 SX Fourier-transform infrared ~FTIR! spec-trometer with 0.25 cm21 resolution in the middle infraredregion.In the HCl/Kr matrices, the HCl monomer bands at2872.9 and 2870.5 cm21 dominate in the spectra ~see thelower spectrum in Fig. 1!.9 HCl molecules were photode-composed at 7.5 K with 193 nm radiation from an ArF-eximer laser ~MPB, MSX-250! by using ;104 pulses with apulse energy density of ;10 mJ/cm2, which led to decompo-sition of .90% of HCl. As reported previously, HKrCl ap-pears as an intermediate during photolysis.10 Annealing ofthe photolyzed HCl/Kr matrix leads to the additional forma-tion of HKrCl molecules, as shown in Fig. 2~a! in the lowesttrace. The H–Kr stretching ~1476.1 cm21! and H–Kr–Clbending ~543.7 and 542.1 cm21 with the overtone at 1069.3and 1067.9 cm21! absorptions are observed, in agreementwith the literature data.1,2In the HCl/N2 /Kr matrices, the known bands of theHClflN2 complex at 2853.6 and 2851.2 cm21 are seen ~theupper spectrum in Fig. 1!.7 Taking into account the fourtimes larger computational IR absorption intensity ofN2-complexed HCl compared with the HCl monomer andintegrated experimental absorptions, the @HCl#/@HClflN2#ratio is estimated to be ;2.5 for HCl:N2 :Kr51:2:1000. Ad-ditionally, we observed the N–N stretch of the HClflN2complex at 2327.4 cm21. The 193 nm radiation decomposedthe HClflN2 complex with a somewhat smaller rate thanmonomeric HCl. Annealing of the photolyzed HCl/N2 /Krmatrix generated, in addition to HKrCl, a set of new bandsdue to N2 doping. In the H–Kr stretching region theN2-induced bands are at 1480.2 ~s!, 1497.7 ~w!, 1508.5 ~s!,1514.5 ~w! and 1589.0 cm21 ~w! @see the upper spectrum inFig. 2~a!#. In the H–Kr–Cl bending region, we observed a© 2002 American Institute of Physics AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp962 J. Chem. Phys., Vol. 117, No. 3, 15 July 2002 Lignell et al.band at 541.5 cm21 ~w! and in the bending overtone region aband at 1067.0 cm21 ~w! @see the upper spectrum in Fig.2~b!#. These N2-induced bands are in the middle trace of Fig.2 where HKrCl monomer bands are subtracted from thecomplex bands. These additional nitrogen-induced bands areproposed to belong to the HKrClflN2 complex.The experimental assignment is based on the followingarguments. Upon annealing, the efficient increase of thecomplex bands starts at ;25 K and the formation saturates at;30 K, similarly to the monomer bands.4 This temperaturedependence fits well the mobility of atomic hydrogen in solidkrypton lattice.11 Both monomer and complex bands are ther-mally stable below the matrix sublimation temperature ~;40K!. The HKrClflN2 bands are observed as intermediate dur-ing photolysis, qualitatively similar to the behavior of themonomeric HKrCl bands.10 Photostability of HKrCl and thenitrogen-induced species are similar to each other as checkedwith 193 nm and 350 nm irradiation ~see Ref. 10 for theHKrCl photodecomposition profile!. Importantly, the decom-position upon irradiation at 350 nm suggests that the com-plex contains HKrCl, since HCl, N2 , or natural matrix im-purities do not decompose at this wavelength. We excludedthe complexation with impurity water as the origin of thecomplex bands by additional experiments using water dop-ing. The ternary N2 complex of HKrCl was also ruled out bythe experiment with the extensive N2 doping.We employed ab initio calculations to study theHKrClflN2 complex. The calculations were carried outwithin the framework of the GAUSSIAN 98 ~revision A.9!package of computer codes.12 The local minima on the inter-molecular potential energy surface were preliminary com-puted at the Hartree–Fock level and using relatively smallbut computationally cost-effective basis sets. For the finalcalculations we employed the Møller–Plesset second orderperturbation theory ~MP2! including all electrons explicitlyinto the correlation calculation. The Berny optimization wasused with the tight convergence criteria. The basis set wasFIG. 1. FTIR spectra of HCl and HCl/N2 in solid Kr at 7.5 K. The HClflN2complex in the lower spectrum originates from the N2 impurity present in amatrix.Downloaded 21 Sep 2007 to 128.214.3.62. Redistribution subject tothe standard split-valence 6-31111G(2d ,2p) which hasbeen found to yield reasonable results for weakly boundcomplexes.13 The vibrational frequencies and IR intensitieswere calculated by using harmonic force fields.Two true energy minima of the HKrClflN2 complex arefound and we call them ~a! linear and ~b! bent ~see Fig. 3!.Computationally, the HKrCl monomer has a dipole momentof 7.4 D. According to the natural population analysis ~NPA!at the MP2 level, HKrCl induces a small dipole moment inthe N2 molecule with partial charges of ~10.061, 20.042!and ~10.020, 20.026! for the linear and bent configurations,respectively. Due to the small induced dipole moment of N2 ,some IR absorption intensity of the N–N stretch is activatedin the complexes. The basis set superposition error ~BSSE!corrected electronic interaction energies for the HKrClflN2complexes are 2471.5 and 2495.1 cm21 for the linear andbent configuration, respectively. The vibrational zero-pointenergy ~ZPE! corrected interaction energies are 2234.2FIG. 2. FTIR spectra of monomeric and complexed HKrCl in solid Kr at 7.5K: ~a! H–Kr stretching region and ~b! H–Kr–Cl fundamental and overtonebending regions. In the middle trace, HKrCl monomer bands are subtracted.The species are prepared by using HCl/Kr and HCl/N2 /Kr matrices, irradi-ating them at 193 nm and annealing at 34 K. Bands labeled A and B areproposed to originate from the linear and bent N2 complexes of HKrCl,respectively. A band marked with C is proposed to originate from a longer-range interaction of HKrCl with N2 . AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp963J. Chem. Phys., Vol. 117, No. 3, 15 July 2002 Blue shift of H–Kr stretchingcm21 for the linear complex and 2366.8 cm21 for the bentcomplex. For comparison, the electronic interaction energiesfor HFflN2 and HClflN2 complexes are 2812.5 cm21 and2334.4 cm21, respectively.14,15In order to discuss the nature of interaction in theHKrClflN2 complex, a simple energy component analysiswas done. We assume that the major part of the interactionenergy is contributed by dipole-quadrupole, dipole-induced-dipole, dispersive and repulsive interactions. The multipolemoment expansion terms were used to calculate the dipole-quadrupole and dipole-induced-dipole interaction.16 The ex-perimental values of the quadrupole moment ~21.468 D Å!and the polarizability ~1.740 Å3! of N2 are taken from theliterature,5 and the dipole moment of HKrCl ~7.4 D! is takenfrom our ab initio computations. The dispersion energy andthe repulsive part is estimated with an anisotropic Lennard-Jones ~6–12! potential function,17 determined for theN2flKr system.18 For the energy component analysis weused MP2(full)/6-31111G(2d ,2p) geometries of com-plexes. In both complexes, the interaction is mainly ofdipole-quadrupole ~39% in linear, 45% in bent! and disper-sion ~54% in linear, 45% in bent! origins. A smaller part ofthe interaction energy is found to originate from the dipole-induced-dipole interaction ~7% in linear, 10% in bent!. Ac-FIG. 3. Computational @MP2(full)/6-31111G(2d ,2p)# structures for twoconfigurations of the HKrClflN2 complex: ~a! linear configuration and ~b!bent configuration. The distances are in Å and the angles in degrees.Downloaded 21 Sep 2007 to 128.214.3.62. Redistribution subject tocording to this energy component analysis, the total interac-tion energies of 2317 cm21 ~linear! and 2581 cm21 ~bent!are in reasonable agreement with our ab initio data.We assign the bands labeled A and B in Fig. 2 to thelinear and bent configurations of the HKrClflN2 complex,respectively. The harmonic vibrational frequencies and theIR absorption intensities are presented in Table I. For thelinear complex, an exceptionally large blueshift of the H–Krstretching mode is found computationally ~1145.7 cm21!and observed experimentally ~1112.9 cm21!. The calculationfor the linear complex predicts a large blueshift of the bend-ing mode ~129.4 cm21!, but a corresponding band was notexperimentally observed. For the bent complex, a large blue-shift is obtained for the H–Kr stretching mode both compu-tationally ~131.0 cm21! and experimentally @132.4 cm21,band B in Fig. 2~a!#. In the H–Kr–Cl bending region, a smallredshift is found computationally ~25.4 cm21 and 22.7cm21! in agreement with the experimental value @22.2cm21, band B in Fig. 2~b!#. The experimental shift for bend-ing overtone of the bent complex ~22.3 cm21! is close tothat for the bending fundamental, which indicates higher har-monicity for the complex bending mode compared with themonomer. A slightly blueshifted ~14.1 cm21! band marked Cin Fig. 2~a! and corresponding bands at bending region @Fig.2~b!# are proposed to belong to the HKrCl perturbed by N2molecule from a longer distance. The origin of the perturba-tion can be ~1! an additional site connected with nitrogen-induced perturbation of the local matrix morphology or ~2!the longer-range ~loose! HKrClflN2 complex. Analogousloose complexation or longer-range interactions are noticedin the experiments with hydrogen peroxide and water.19,20The formation of the bent HKrClflN2 complex followslinearly the amount of the photolyzed HClflN2 . This indi-cates that the bent complex originates from the ClflN2 com-plexes formed in photolysis. The linear complex does notcorrelate directly with the amount of photolyzed HClflN2but shows linear dependence with the product of @photolyzedHCl#@N2# , indicating that the linear complex is formed fromthe photolyzed HCl monomer having N2 as a close neighbor.The large spectral blueshifts of the H–Kr stretchingmode are attributed to the enhanced (HKr)1Cl2 ion-paircharacter of complexed HKrCl, resulting in stronger H–Krcovalent bonding. According to the NPA, the partial chargeof the HKr entity of the HKrCl monomer is 10.673. UponTABLE I. Computational and experimental vibrational data of the HKrClflN2 and HClflN2 complexes and HKr1 ion. The experimental monomer-to-complex shifts are calculated from the strongest monomer and complex bands. The IR absorption intensities are marked in parentheses. Frequencies are incm21 and intensities are in km mol21. The calculations correspond to the 35Cl isotope.N21HKrCl n~H–Kr! d~H–Kr–Cl! n~Kr–Cl! n~N–N! N21HCl n~H–Cl! HKr1 n~H–Kr!MP2(full)/6-31111G(2d ,2p)Monomer 1828.4~2412! 600.3~38! 278.5~122! 2179.4~0! 2998.1~51.8! 2600.4~401!Bent complex 131.0~2189! 25.4~19!; 22.7~21! 23.3~119! 20.4~0.3!Linear complex 1145.7~907! 129.4~21!; 129.4~21! 211.3~141! 21.6~2.7! 224.1~199!ExperimentalMonomer 1476.1 543.7, 542.1, 1069.3a, 1067.9a - - 2872.9 2429.2bBent complex 132.4 22.2, 22.3a - -Linear complex 1112.9 - - - 219.3aOvertone.bReference 21. AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp964 J. Chem. Phys., Vol. 117, No. 3, 15 July 2002 Lignell et al.complexation with N2 , the corresponding partial charges are10.709 and 10.685 for the linear and bent configurations,respectively. We can speculate that, while the (HKr)1Cl2ion-pair character increases, the H–Kr vibrational frequencyand absorption intensity approach the corresponding valuesof the HKr1 ion with higher frequency and lower absorptionintensity ~see Table I!.21 The computational value for theH–Kr stretching frequency of HKr1 @MP2(full)/6-31111G(2d ,2p)# is 2600.4 cm21 with intensity of 401km mol21. As a comparison, the corresponding values of theHKrCl monomer obtained at the same level are 1828.4 cm21and 2412 km mol21.In summary, two HKrClflN2 configurations are charac-terized experimentally in solid krypton and by ab initio cal-culations. The computational structures are presented in Fig.3. Due to the complexation-enhanced ion-pair character of(HKr)1Cl2, the complexes have large spectral blueshifts ofthe H–Kr stretching mode compared with the monomer. Theexperimental shifts are in agreement with the computations.The interaction energy of the complexes originates mostlyfrom dipole-quadrupole and dispersive interactions.This work was supported by the Academy of Finland.The CSC-Center of Scientific Computing Ltd. ~Espoo, Fin-land! is thanked for providing computer facilities. Dr. JanLundell is gratefully thanked for very useful discussions.A. L. is a member of the graduate school LASKEMO ~Acad-emy of Finland!.Downloaded 21 Sep 2007 to 128.214.3.62. Redistribution subject to1 M. Pettersson, J. Lundell, and M. Ra¨sa¨nen, J. Chem. Phys. 102, 6423~1995!.2 J. Lundell, L. Khriachtchev, M. Pettersson, and M. Ra¨sa¨nen, Low Temp.Phys. 26, 680 ~2000!.3 L. Khriachtchev, M. Pettersson, A. Lignell, and M. Ra¨sa¨nen, J. Am. Chem.Soc. 123, 8610 ~2001!.4 M. Pettersson, L. Khriachtchev, A. Lignell, M. Ra¨sa¨nen, Z. Bihary, and R.B. Gerber, J. Chem. Phys. 116, 2508 ~2002!.5 CRC Handbook of Chemistry and Physics, 72nd ed. ~CRC, Cleveland,1991!.6 J. 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Large blueshift of the H–Kr stretching frequency of HKrCl upon complexation with N₂

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