A. Shayesteh et al- The vibration–rotation emission spectrum of MgH2

8/2/2019 A. Shayesteh et al- The vibrationrotation emission spectrum of MgH2

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The vibrationrotation emission spectrum of MgH2A. Shayesteh and D. R. T. Appadoo

Department of Chemistry, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada

I. GordonDepartment of Physics, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada

P. F. Bernatha)

Department of Chemistry and Department of Physics, University of Waterloo, Waterloo, Ontario,

N2L 3G1, Canada

Received 27 June 2003; accepted 25 July 2003

The gaseous MgH2 molecule has been discovered in an electrical discharge inside a hightemperature furnace. The vibrationrotation emission spectrum of 24MgH2 was recorded with aFourier transform spectrometer and the antisymmetric stretching mode (3) was detected near 1589cm1. In addition, three hot bands involving 2 and 3 were found and rotationally analyzed. TheMgH2 molecule has a linear structure with an R0 MgH bond distance of 1.703 3273 . 2003 American Institute of Physics. DOI: 10.1063/1.1609973

INTRODUCTION

The reaction of magnesium vapor with hydrogen has

been studied many times in the last two decades.1 6 It isknown that Mg atoms in the 3 s3p excited electronic states(3P and 1P) can activate the HH bond,5 and the MgH2molecule is assumed to be a reaction intermediate in theproduction of MgH in the following reaction:

MggH2 gMgHgH g.

This reaction has been studied extensively by pumpprobelaser experiments4 6 as well as by far wing laserscattering.2,3 Breckenridge has recently reviewed the experi-mental and theoretical status of the reactivity of Mg, Zn, Cd,and Hg with HH, SiH, and CH bonds.5 The reaction ofground state 1S magnesium with H

2is inhibited by a large

barrier to insertion. The ground state reaction,

Mg 1S H2gMgH2g,

is calculated to be endoergic by 5 kcal/mol7 or 3 kcal/mol8

depending on the level of theory used. In fact, Ahlrichset al.8 state that ... we conclude that MgH2 is not boundwith respect to MgH2 and hence not likely to be a stablespecies. We report here on the discovery of free gaseousMgH2 .

As MgH2 is a relatively small molecule, it has been stud-ied several times by ab initio methods.711 For example,Tschumper and Schaefer calculated the molecular geometry

and the harmonic vibrational frequencies of MgH2 usinghigh level ab initio methods with large basis sets.11 In spiteof this interest in MgH2 , the molecule remains unknownexcept for the detection of its infrared spectrum in argon,12

krypton, and xenon matrices13 at 1012 K.Solid MgH2 is well known.

14 Molecular hydrogen is eas-ily released by heating the solid MgH2 to 85 C.

14 This heat-ing results in decomposition to the elements, not the produc-

tion of gaseous MgH2 . MgH2 and related materials havebeen proposed for hydrogen storage because of their low cost

and large hydrogen-storage capacity.15,16

Recently we reported on our detection of gaseousBeH2 ,

17,18 and we now present results on MgH2 . The onlyknown gaseous metal dihydrides are BeH2 ,

17,18 MgH2 , andFeH2 .

19

EXPERIMENTAL DETAILS

The high resolution vibrationrotation emission spec-trum of MgH2 was recorded with a Fourier transform spec-trometer. The MgH2 molecule was generated in an emissionsource that combines an electrical discharge with a high tem-perature furnace. The central part of an alumina tube (5 cm

120 cm) was heated to 650 C by a CM Rapid Temp fur-nace. The ends of the tube were cooled by water and sealedwith CaF2 windows. About 20 g of powdered magnesiummetal was placed in the center of the tube, and a slow flow ofargon 1.6 Torr and hydrogen 0.9 Torr was passed throughthe cell. A dc discharge 3 kV, 333 mA was struck betweentwo stainless steel tube electrodes inside the cool ends of thealumina tube. The emitted light from the source was focusedwith a CaF2 lens to the entrance aperture of a Bruker IFS 120HR Fourier transform spectrometer. The spectrum was re-corded using a CaF2 beamsplitter and a liquid nitrogen-cooled HgCdTe MCT detector at an instrumental resolutionof 0.01 cm1. The spectral region was limited to 12002200

cm

1 by the CaF2 beamsplitter and a 2200 cm

1 long-wavepass filter. Approximately 550 scans were added during 8 hof integration. The recorded spectrum contained atomic andmolecular emission lines, as well as absorption lines fromatmospheric water vapor.

RESULTS AND ANALYSIS

The strongest emission lines in the spectrum are fromthe v10 fundamental band of MgH, for which the bandhead is at 1578.8 cm1. In addition, a series of weaker lines

aAuthor to whom all correspondence should be addressed. Electronic mail:[email protected]

JOURNAL OF CHEMICAL PHYSICS VOLUME 119, NUMBER 15 15 OCTOBER 2003

77850021-9606/2003/119(15)/7785/4/$20.00 2003 American Institute of Physics

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with alternating 3:1 intensities were observed to higher wavenumbers Fig. 1, which are due to the antisymmetric stretch-ing fundamental band of 24MgH2 .

The program WSPECTRA written by Carleer UniversiteLibre de Bruxelles was used to determine the line positions.The spectrum was calibrated using impurity CO lines, andthe absolute accuracy of the measured lines is better than0.001 cm1. A color Loomis-Wood program was used as anaid for the assignment of the observed bands.

The lines of Table I are assigned to the 3 (u) antisym-metric stretching mode of the linear MgH2 molecule for thefollowing reasons:

1 For the linear MgH2 molecule with Dh symmetry, theadjacent rotational lines have a 3:1 intensity ratio due tothe orthopara nuclear spin statistical weights (I1) /Iassociated with the I 12 hydrogen nuclei,

20 which isconsistent with our observation.

2 There are three measurements of the band origin of the3 mode of MgH2 in argon, krypton, and xenon matricesthat predict the band origin at 1571.9, 1558, and 1569cm1, respectively.12,13 The band origin of 1588.672cm1 observed in our spectrum matches these measure-ments if matrix shifts are taken into account.

3 A high level ab initio calculation11 predicted the MgH

equilibrium distance (R

e) at 1.7108 . The observedvalue ofR0 in our experiment is 1.703 3273 , whichis in good agreement with the theoretical prediction.

In addition to the 3 fundamental band, 001000 u

g , three hot bands of MgH2 were also assigned and

rotationally analyzed:

011010 gu , 002001 g u

,

0221f 0220f u g .

The absolute rotational assignment of the 001000,u g

, fundamental band was made based on the miss-ing line at the band origin and the 3:1 intensity alternation.

The next strongest band had a large l-type doubling, and wasassigned to the first bending mode hot band, 011010,g u transition. In this transition, the absolute rotational

assignment was made based on the fact that e and f paritylevels have the same band origins. We used the energy levelexpression:

Ev1 ,v2 ,v3 ,JGv1 ,v2 ,v3FJ, 1

with

FJBJJ1 DJ2J12HJ3J1 3

12 qJJ1 qDJ

2J12 2

in our least-squares fitting program. The constants of Table IIwere determined for 24MgH2 using Eq. 2, in which qqD0 for states, and the plus minus sign refers to

e(f ) parity for states. The observed line positions and theoutputs of the least-squares fits for all the bands have beenplaced in Electronic Physics Auxiliary Publication ServiceEPAPS.21 By fitting the 001000 and 011010 bands, wedetermined two vibrationrotation interaction constants, 2and 3 , using

20

Bv1 ,v2 ,v3

B e1v112 2v213v3

12 .

3

For the other hot bands, 002001 and 021020, fewer lineswere observed and the absolute rotational assignments weremore difficult. The B

v1v2v3values of these vibrational levels

were predicted from B000 and values. Different absolute

FIG. 1. An expanded view of the R branch of the 001000 band of24MgH2 . The MgH2 lines are marked with asterisks and display a 3:1 in-tensity alternation. The absorption lines are from atmospheric water vapor.

TABLE I. The line positions of the 001000, u g

, band of 24MgH2in cm1.

Assignment Line posi tion Assignment Line posit ion

R29 1726.150143a P1 1582.90650R27 1719.33865 P2 1577.07387R25 1712.16264 P3 1571.17672R23 1704.63195 P4 1565.215520R22 1700.738943 P5 1559.18674

R21 1696.75134 P6 1553.0933

29R20 1692.679417 P7 1546.94458R19 1688.52634 P8 1540.72929R18 1684.28646 P9 1534.45695R17 1679.960711 P10 1528.124411R16 1675.556520 P11 1521.73163R15 1671.06452 P12 1515.28385R14 1666.491914 P14 1502.21764R13 1661.84091 P15 1495.60294R12 1657.106615 P16 1488.93607R11 1652.29591 P17 1482.213811R9 1642.43743 P18 1475.444313R8 1637.390910 P19 1468.62061R7 1632.27036 P20 1461.74922R6 1627.07460 P21 1454.82882R5 1621.80476 P22 1447.86101

R4 1616.461312 P23 1440.84727R3 1611.04414 P25 1426.68190R2 1605.558426 P27 1412.340818R1 1599.99638 P31 1383.17066

P33 1368.35258

aObserved minus calculated values computed with the constants of Table IIin units of 104 cm1).

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rotational assignments were attempted for these two bands,and in each band only one rotational assignment resulted inB

v1v2v3values close to the predicted ones. The 001000 and

002001 bands were fitted together, and that also confirmedour assignment.

We observed a large splitting between e and f levels ofthe 0220 (g) and the 02

21 (u) vibrational levels, which isdue to the l-type resonance with the 0200 (g

) and the

02

0

1 (u

) levels, respectively.

2225

Since all the

rota-tional levels have e parity, they interact only with the statee levels, and therefore the f energy levels are unaffected.In our previous work on BeH2 , we performed a more com-plete analysis of the l-type resonance by using a Hamiltonianmatrix that included the appropriate interaction matrixelements.18 However, the number of observed lines forMgH2 was smaller and the data were not sufficient to deter-mine the desired constants. Only the f parity of the states,0221 (f )0220 (f ) band, was fitted using a simple stateenergy expression, and the constants are presented in TableII.

DISCUSSION

The vibrational energy G(v1 ,v2 ,v3) in Eq. 1 can bewritten as20

Gv1 ,v2 ,v31v112 2v213v3

12

x11v112

2x22v21

2x33v3

12

2

x12v112 v21x13v1

12 v3

12

x23v21v312 g22l2

2. 4

The vibrational energies in Table II are presented as the dif-ference between G(v1 ,v2 ,v3) and the zero point energy

ZPE, G( 0,0,0) . We estimated the value of G(0,1,0) G(0,0,0) in Table II (2) using

24,25

q0102Be

2

2 1 42

2

322

2 . 5Equation 5 is for equilibrium constants, which are notavailable, so we employed B0 , 2 , and 3 instead of B e ,2 , and 3 , respectively. Equation 5 results in a value of

437 cm1

for 2 which is close to the matrix isolation valueof 439.8 cm1.12 The MgH bond distance (R0) was calcu-lated from B0 to be 1.703 3273 , in agreement with the abinitio value of 1.7108 .11 We were also able to determinex23 and x33 in Eq. 4 using the 001000, 011010, and002001 band origins Table III.

Although BeH2 , MgH2 , and the dihydrides of group 12elements (ZnH2 , CdH2 , and HgH2) are linear,

26 the heavierdihydrides of the group 2 elements (CaH2 , SrH2 , and BaH2)may be bent.27 A very recent ab initio calculation has pre-dicted the HMH bond angles of CaH2 , SrH2 , and BaH2to be 180, 136, and 123, respectively.9 There are other abinitio calculations predicting different bond angles for these

molecules.28,29 In fact, Xiao et al. recorded the infrared spec-trum of CaH2 in krypton and xenon matrices at 12 K, andthey assigned a very weak vibrational band to the symmetricstretching mode of CaH2 assuming that it is bent.

30

We have performed experiments similar to our BeH2 andMgH2 work trying to record the vibrationrotation emissionspectra of group 2 and 12 dihydrides in the gas phase todetermine their structures. So far we have found gaseousZnH2 with a linear structure.

31 We also recorded the infraredemission spectra of CaH, SrH, and BaH with high signal-to-noise ratios, but we did not see any sign of CaH2 , SrH2 , orBaH2 .

In summary, we observed the gaseous MgH2 molecule

for the first time. The antisymmetric stretching mode (3)and a few hot bands were rotationally analyzed. The pre-dicted linear structure of MgH2 with Dh symmetry wasconfirmed and the R0 MgH bond distance was determined.

ACKNOWLEDGMENT

This work was supported by the Natural Sciences andEngineering Research Council NSERC of Canada.

1 N. Adams, W. H. Breckenridge, and J. Simons, Chem. Phys. 56, 3271981.

2 P. D. Kleiber, A. M. Lyyra, K. M. Sando, S. P. Heneghan, and W. C.Stwalley, Phys. Rev. Lett. 54, 2003 1985.

TABLE II. Spectroscopic constants of 24MgH2 in cm1 all uncertainties are 1.

Level GvZPE B 105 D 1010 H 102 q 106 qD

000 g 0.0 2.882 60711 3.917825 6.4917

001 u 1588.671 5724 2.848 66711 3.891928 6.5321

002 g 3165.420 0252 2.815 07813 3.866439 6.5735

010a u a 2.891 55715 4.086547 7.2045 5.046019 3.23929011 g a1582.695 60(26) 2.857 49915 4.058150 6.8452 4.997620 3.21233

0220(f ) g b 2.900 03721 4.171128

022

1(f ) u b

1576.706 89(63) 2.865 91321 4.151531aThe estimated value ofa (2) is 437 cm

1, and the value of b is approximately 2 times 2 see the text.

TABLE III. Molecular constants of 24MgH2 in cm1.

R0 1.703 326 731

B0 2.882 607112 0.008 950183 0.033 94015

q010 0.050 460192(u) 437

a

3(u) 1588.671 5724x23 5.975 9735x33 5.961 5635

aCalculated from the q010 value and Eq. 5.

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3 P. D. Kleiber, A. M. Lyyra, K. M. Sando, V. Zafiropulos, and W. C.Stwalley, J. Chem. Phys. 85, 5493 1986.

4 W. H. Breckenridge and J. H. Wang, Chem. Phys. Lett. 137, 195 1987.5 W. H. Breckenridge, J. Phys. Chem. 100, 14840 1996.6 D. K. Liu and K. C. Lin, Chem. Phys. Lett. 274, 37 1997.7 J. A. Pople, B. T. Luke, M. J. Frisch, and J. S. Binkley, J. Phys. Chem. 89,

2198 1985.8 R. Ahlrichs, F. Keil, H. Lischka, W. Kutzelnigg, and V. Staemmler, J.

Chem. Phys. 63, 455 1975.9 L. von Szentpaly, J. Phys. Chem. A 106, 11945 2002.

10

P. Chaquin, A. Sevin, and H. Yu, J. Phys. Chem. 89, 2813 1985.11 G. S. Tschumper and H. F. Schaefer, J. Chem. Phys. 108, 7511 1998.12 T. J. Tague, Jr. and L. Andrews, J. Phys. Chem. 98, 8611 1994.13 J. G. McCaffrey, J. M. Parnis, G. A. Ozin, and W. H. Breckenridge, J.

Phys. Chem. 89, 4945 1985.14 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements Perga-

mon, Oxford, 1984, p. 72.15 J. Huot, G. Liang, and R. Schulz, Appl. Phys. A: Mater. Sci. Process. 72,

187 2001.16 S. Bouaricha, J. Huot, D. Guay, and R. Schulz, Int. J. Hydrogen Energy27, 909 2002.

17 P. F. Bernath, A. Shayesteh, K. Tereszchuk, and R. Colin, Science 297,1323 2002.

18 A. Shayesteh, K. Tereszchuk, P. F. Bernath, and R. Colin, J. Chem. Phys.118, 3622 2003.

19 H. Korsgen, W. Urban, and J. Brown, J. Chem. Phys. 110, 3861 1999.

20 P. F. Bernath, Spectra of Atoms and Molecules Oxford University Press,New York, 1995.

21 See EPAPS Document No. E-JCPSA6-119-015339 for the observed linepositions and the outputs of the least-squares fits for all the bands. A directlink to this document may be found in the online articles HTML referencesection. The document may also be reached via the EPAPS homepagehttp://www.aip.org/pubservs/epaps.html or from ftp.aip.org in the direc-tory /epaps/. See the EPAPS homepage for more information.

22 G. Amat and H. H. Nielsen, J. Mol. Spectrosc. 2, 163 1958.23

A. G. Maki, Jr. and D. R. Lide, Jr., J. Chem. Phys.47

, 3206 1967.24 D. Papousek and M. R. Aliev, Molecular Vibrational-Rotational SpectraElsevier, Amsterdam, 1982.

25 J. K. G. Watson, Can. J. Phys. 79, 521 2001.26 T. M. Greene, W. Brown, L. Andrews, A. J. Downs, G. V. Chertihin, N.

Runeberg, and P. Pyykko, J. Phys. Chem. 99, 7925 1995.27 S. Aldridge and A. J. Downs, Chem. Rev. 101, 3305 2001.28 I. Bytheway, R. J. Gillespie, T. H. Tang, and R. F. W. Bader, Inorg. Chem.34, 2407 1995.

29 M. Kaupp, P. v. R. Schleyer, H. Stoll, and H. Preuss, J. Chem. Phys. 94,1360 1991.

30 Z. L. Xiao, R. H. Hauge, and J. L. Margrave, High. Temp. Sci. 31, 591991.

31 A. Shayesteh, D. R. T. Appadoo, I. Gordon, and P. F. Bernath unpub-lished.

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A. Shayesteh et al- The vibration–rotation emission spectrum of MgH2

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