Spectroscopy and calculations of weakly bound gallium complexes with ammonia and monomethylamine

Article abstract of DOI:10.1063/1.1410973

The adiabatic IPs and intermolecular vibrational frequencies were obtained for the weakly bound gallium-ammonia and -monomethylamine complexes. It was found that the methyl substitution for hydrogen in ammonia enhances the binding of the gallium atom and

Full text of DOI:10.1063/1.1410973

Spectroscopy and calculations of weakly bound gallium complexes with ammonia and
monomethylamine
Shenggang Li, Gretchen K. Rothschopf, Dinesh Pillai, Bradford R. Sohnlein, Benjamin M. Wilson, and Dong-
Sheng Yang
Citation: The Journal of Chemical Physics 115, 7968 (2001); doi: 10.1063/1.1410973
View online: http://dx.doi.org/10.1063/1.1410973
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 115, NUMBER 17
1 NOVEMBER 2001
Spectroscopy and calculations of weakly bound gallium complexes
with ammonia and monomethylamine
Shenggang Li, Gretchen K. Rothschopf, Dinesh Pillai, Bradford R. Sohnlein,
Benjamin M. Wilson, and Dong-Sheng Yanga)
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055
͑Received 12 July 2001; accepted 23 August 2001͒
The gallium complexes were produced in pulsed molecular beams and studied with zero electron
kinetic energy ͑ZEKE͒ photoelectron spectroscopy. Intermolecular vibrational frequencies and
adiabatic ionization potentials ͑IPs͒ were obtained from ZEKE spectra. Ground electronic states
were identified by combining the ZEKE spectra with quantum chemical and Franck–Condon
calculations. Ga–NH3 has an IP of 40 135 cmϪ1 and metal-ligand stretching frequencies of 270
cmϪ1 (␻ϩ) in the ion and 161 cmϪ1 (␯s) in the neutral. The IP of Ga–NH2CH3 is 39 330 cmϪ1, and
s
the vibrational frequencies are 93 cmϪ1 (␯b) for the Ga–N–C bending, 124 cmϪ1 (␻bϩ) for the
Gaϩ–N–C bending, and 299 cmϪ1 (␻ϩ) for the Gaϩ–N stretching. The strength of the gallium–
s
methylamine binding is stronger than that of the gallium–ammonia. The ground state of Ga–NH3
is 2A (C ) and that of Gaϩ–NH is 1A (C ). In contrast, Ga–NH CH has two doublets, 2A and
Ј
Ј
s
3
1
3v
2
3
1
2A (C ), with virtually the same energies, whereas Gaϩ–NH CH has a A (C ) ground state.
Љ
Ј
s
2
3
s
© 2001 American Institute of Physics. ͓DOI: 10.1063/1.1410973͔
studies of group 13 metal clusters with ammonia.17
I. INTRODUCTION
Zero electron kinetic energy ͑ZEKE͒ photoelectron spec-
troscopy is a highly informative method for characterizing
weakly bound complexes consisting of metal atoms ͑or clus-
ters͒ and molecular ligands.18–23 Here we report on the first
ZEKE study of Ga–NH3 and Ga–NH2CH3, in combination
with density functional theory ͑DFT͒ and ab initio calcula-
tions. This study has provided spectroscopic evidence for the
formation of the weakly bound gallium–ammonia and
–monomethylamine complexes and determined adiabatic
ionization potentials ͑IPs͒ and intermolecular vibrational en-
ergy levels for these species. The study of the Ga–NH2CH3
complex has revealed methyl substitution effects on elec-
tronic spectra and metal-ligand binding in these species.
Gallium nitride ͑GaN͒ has emerged as a potential mate-
rial for making blue and UV light emitters and high tempera-
ture, high power electronic devices.1–3 GaN thin films have
been prepared by chemical vapor deposition techniques.4
These techniques involve reactions of volatile gallium
compounds5,6 or gallium atomic vapors7,8 with nitridating
agents, such as nitrogen, ammonia, and amines, or decompo-
sitions of single-source precursor compounds.9 The transpor-
tation and reaction kinetics of gaseous species have been
proposed to play an important role in GaN film properties
and growth rates.4,5
Recently, several groups studied reactions of trimethyl-
gallium and gallium atom with nitrogen-containing mol-
ecules using matrix-isolation infrared spectroscopy.10–15 Pio-
cos and Ault,10 Edgar et al.,11 and Almond et al.12 reported
the formation of ͑CH3͒3Ga–NH3 and ͑CH3͒3Ga–N͑CH3͒3
from reactions of trimethylgallium vapor with ammonia and
trimethylamine gases. Zhou and Andrews reported GaN,
NGaN, Ga3N, and GaN3 formed by laser-ablated gallium
atoms reacting with nitrogen.13 Himmel et al. reported
Ga–NH3, HGaNH2, GaNH2, and H2GaNH2 formed by reac-
tions of Knudsen-cell vaporized Ga with NH3.14,15 However,
Ga–NH3 intermolecular vibrations were not observed in
their studies, as these modes fall below the low-frequency
threshold of detection in their experiments. In addition,
Greetham et al. measured laser-induced fluorescence spectra
of Ga–N2.16 The Ga–N2 complex was formed by laser abla-
tion of a GaAs or GaP target, followed by supersonic expan-
sion in pure nitrogen. Di Palma et al. reported a photoioniza-
tion efficiency curve of Ga–NH3 in their photoionization
II. EXPERIMENTAL AND COMPUTATIONAL METHODS
The molecular beam ZEKE apparatus has been de-
scribed in a previous publication.19 Gallium complexes were
produced by reactions of laser-vaporized gallium atoms with
ammonia and monomethylamine. Gallium ablation target
was prepared by coating the metal ͑99.99%, Aldrich͒ onto a
glass rod. The target was rotated and translated by a motor-
driven mechanism and vaporized with the second harmonic
output of a Nd:YAG laser ͑2 mJ, 532 nm, Quanta-Ray GCR-
3͒. Ammonia ͑99.9%, Metheson͒ and monomethylamine
͑99.5%, Metheson͒ were mixed with helium gas ͑UHP,
Scott-Gross͒ in 2% concentration and delivered to the vapor-
ization region by a piezoelectric pulsed valve, with a backing
pressure of 40 psi. The helium-reactant pulses were synchro-
nized with the vaporization laser pulses to optimize the
metal-molecule reactions.
Masses of the products formed in the molecular beams
were determined by photoionization time-of-flight ͑TOF͒
mass detection. Approximate IPs of Ga–NH3 and
a͒
Author to whom correspondence should be addressed; electronic-mail:
dyang0@pop.uky.edu. Fax: ͑859͒ 323-1069.
0021-9606/2001/115(17)/7968/7/$18.00
7968
© 2001 American Institute of Physics
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J. Chem. Phys., Vol. 115, No. 17, 1 November 2001
Gallium complexes with ammonia
7969
GaNH2CH3 were measured by threshold photoionization
measurements in the TOF mass operation mode. Prior to the
ZEKE experiments, the peaks of Ga–NH3 and Ga–NH2CH3
in the mass spectra were maximized by optimizing the tim-
ing and power of the ablation and ionization lasers, the con-
centration of the reactants, and the backing pressure of the
helium gas. The ZEKE measurements were carried out by
first photoexcitation of the molecules to high-lying Rydberg
levels and then pulsed-field ionization of the Rydberg states.
The second harmonic output of a dye laser ͑Lumonics HD-
500͒, pumped by a XeCl excimer laser ͑Lumonics PM-884͒,
was used as photoionization and photoexcitation sources. A
delay pulse generator ͑Stanford SR 445͒ was used to provide
electric pulses ͑1.2 V cmϪ1, 100 ns͒ for field ionization. The
ZEKE electron signal was capacitively decoupled from the
dual microchannel plate anode, amplified by a preamplifier
͑Stanford SR445͒, averaged by a gated integrator ͑Stanford
SR250͒, and collected and stored in a computer. A small
residual dc field ͑0.08 V cmϪ1͒ was applied to help remove
prompt electrons from direct photoionization. A field depen-
dence study was not performed due to the limited ZEKE
signal. The laser wavelengths were calibrated against vana-
dium atomic transitions in the frequency range of the indi-
vidual spectrum.
FIG. 1. Threshold photoionization spectra of Ga–NH3 ͑a͒ and Ga–NH2CH3
͑b͒. Wave numbers are corrected by ϩ110 cmϪ1 for the shift induced by the
dc extraction field of 320 V cmϪ1
.
DFT and ab initio calculations were performed using the
GAUSSIAN 98 program package.24 Geometries and harmonic
vibrational frequencies of the neutral and ion were calculated
with B3LYP and B3P86 methods. Single point energies at
B3LYP geometries were calculated with coupled-cluster cal-
culations including single and double excitations, per-
turbatively corrected for triples counterpoise-corrected
͓CCSD͑T͔͒ method. For Ga–NH2CH3, coupled-cluster cal-
culations including double excitation ͑CCD͒ were also used
to perform full geometry optimizations. All calculations were
carried with 6-311ϩG(d,p) basis. Multidimensional
Franck–Condon ͑FC͒ factors were computed using the theo-
retical equilibrium geometries and normal modes of the neu-
tral and the ion. In these calculations, the Duschinsky25 effect
was considered to account for normal mode differences in
the neutral and ionic molecules. Details of the FC calcula-
tions were previously described.26,27 Spectral broadening was
simulated by giving each line a Lorenztian line shape with
the linewidth of the experimental spectra ͑6 cmϪ1 for
Ga–NH3, 8 cmϪ1 for Ga–NH2CH3͒. Boltzmann factors were
used to simulate spectra at finite temperatures.
photoionization spectrum for Ga–NH3, but the weak signal
below 40 150 cmϪ1 was not clearly shown in their study.17
The Gaϩ–NH2CH3 signal begins at 39 200 ͑50͒ cmϪ1 and
rises rapidly at 39 320 ͑50͒ cmϪ1. As in the case of Ga–NH3,
the signal around 39 200 cmϪ1 may be due to hot transitions
and the signal above 39 300 cmϪ1 is largely from cold tran-
sitions. In both spectra, a vibrational assignment is difficult
because of the limited resolution. The sharp ionization on-
sets, however, yield approximate IPs, 40 150 ͑50͒ and 39 320
͑50͒ cmϪ1, for these two complexes.
B. ZEKE spectra and vibrational analysis
1. Gallium–ammonia
Figure 2͑a͒ shows the ZEKE spectrum of Ga–NH3. It
consists of a vibrational progression originating at 40 135
cmϪ1, with intervals of around 270 cmϪ1. Table I lists the
positions of these transitions, along with the energy of an
additional peak, a, located 161 cmϪ1 to the red of the band
origin. The progression was fitted to the equation G(␷ϩ)
s
ϭ␻ϩ(␷ϩϩ1/2)ϩxϩs (␷ϩϩ1/2)2. This fit yielded a harmonic
III. RESULTS AND DISCUSSION
s
s
s
frequency of ␻ϩϭ270 (1) cmϪ1 and an anharmonic constant
s
A. Threshold photoionization spectra
xϩs ϭϪ2.3 (3) cmϪ1. The intensity of the peak a varied with
the conditions of the vaporization source, indicating that it is
a hot band. Because NH3 has no modes with a frequency
below 500 cmϪ1, the observed progression must be due to an
intermolecular vibration. Compared to the ZEKE spectra of
Al–NH3 ͑Ref. 18͒ and In–NH3,19 this vibration is expected
to be the symmetric Gaϩ–N stretching as discussed below.
Electronic structure and FC calculations have been used
to assign the ZEKE spectra. Table II summarizes the B3P86
and B3LYP geometries and vibrational frequencies for
Figure 1 presents the threshold photoionization spectra
of 69Ga–NH3 ͑a͒ and 69Ga–NH2CH3 ͑b͒. The spectra of
71Ga–NH3 and 71Ga–NH2CH3 are similar to those of the
69Ga species. The Gaϩ–NH3 signal appears at 40 000 ͑50͒
cmϪ1, and a steep onset begins at 40 150 ͑50͒ cmϪ1. The
small signal between 40 000 and 40 150 cmϪ1 originates
from an excited neutral level, as confirmed by the ZEKE
measurements. Above 40 150 cmϪ1, the spectrum exhibits a
number of steps, associated with various vibrational levels in
Gaϩ–NH3. Previously, Di Palma et al. reported a similar
Ga–NH and Gaϩ–NH . The ground state of Ga–NH is 2A
Ј
3
3
3
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7970
J. Chem. Phys., Vol. 115, No. 17, 1 November 2001
Li et al.
TABLE I. ZEKE peak positions ͑cmϪ1͒ and vibrational assignments. The
ϩ
s
ϩ
absolute uncertainty is ϳ5 cmϪ1 and the relative uncertainty ϳ1 cmϪ1. ␷
and ␷ are the vibrational quanta of the Gaϩ–N and Ga–N stretching, ␷
s
b
and ␷ the vibrational quanta of the Gaϩ–N–C and Ga–N–C bending, and
b
ϩ
␷
and ␷ the vibrational quanta of the NH2 rocking in Gaϩ–NH2CH3 and
r
r
Ga–NH2CH3. The mode descriptions are approximate.
Ga–NH3
Ga–NH2CH3
ϩϪ␷
␷
ϩϪ␷
␷
ϩϪ␷
0Ϫ1
␷
ϩϪ␷
r
r
Position
39 974
40 135
40 401
40 662
40 919
position
39 237
39 274
39 330
39 457
39 491
39 532
39 584
39 625
39 712
39 750
39 783
39 823
39 845
39 875
39 916
40 003
40 039
40 164
40 203
␷
s
s
b
s
s
b
0Ϫ1
0Ϫ0
1Ϫ0
2Ϫ0
3Ϫ0
1Ϫ2
0Ϫ0
1Ϫ0
2Ϫ1
0Ϫ1
2Ϫ0
0Ϫ0
1Ϫ0
1Ϫ0
3Ϫ0
1Ϫ0
2Ϫ1
0Ϫ1
1Ϫ0
1Ϫ0
2Ϫ0
1Ϫ0
1Ϫ0
2Ϫ0
1Ϫ0
2Ϫ0
2Ϫ0
3Ϫ0
2Ϫ0
3Ϫ0
1Ϫ0
2Ϫ0
2
ion, and those in the A state have fair agreement with the
Ј
measured value of ␯sϭ161 cmϪ1 in the neutral. The ZEKE
measurement has corrected a previously reported frequency
of 350 cmϪ1 assigned to the Gaϩ–N stretching mode.17 The
early frequency was incorrect due to a poorly resolved
photoionization spectrum, which made its accurate measure-
ment difficult. All other symmetric modes are predicted to be
much larger than the stretch mode. Vibronic intensities of the
FIG. 2. Experimental ͑a͒ and simulated ͑b–d͒ spectra of gallium-ammonia.
The simulations ͑b,c͒ are for the 1A1(Gaϩ–NH3,C3v 2A (Ga–NH ,C )
transition of the association isomer calculated with the B3P86 and B3LYP
methods, whereas the simulation ͑d͒ for the 1A1(HGaϩNH2,C2v
2A (HGaNH ,C ) transition of the insertion isomer from the B3P86 cal-
)
Ј
3
s
)
Ј
2
s
culations. All simulations were carried out with the temperature of 100 K.
1A1 2A transition have been calculated using the DFT
Ј
equilibrium geometries, harmonic frequencies, and normal
coordinates. The metal-ligand stretching is predicted to be
the only FC active mode, as shown by spectral simulations in
Figs. 2͑b͒ and 2͑c͒. In these simulations, the theoretical IP
has been shifted so that it corresponds to the measured value.
The excellent agreement between the calculated and mea-
sured spectrum confirms the assignment of the metal-ligand
stretch vibration. In comparing the two calculations, the
in Cs symmetry. The highest occupied molecular orbital
͑HOMO͒ is predicted to be a mixture of 71% of Ga ͑ p͒, 5%
of N ͑ p͒, and 24% of H ͑s͒. The Ga p orbital is perpendicular
to the Ga–N axis and mixed in an anti-bonding ␲ fashion
with the ligand orbitals. The ground state of Gaϩ–NH3 is
1
1A1 in C3v symmetry. The HOMO of the A1 state consists
of 39% of Ga ͑sp͒, 52% of N ͑sp͒, and 9% of H ͑s͒, also with
an anti-bonding mixing. The low symmetry of the neutral
state is the result of Jahn–Teller distortion, as in Al–NH3
and In–NH3. The distortion, however, is quite small, as in-
dicated by the variation of the ЄH–N–H angles, which are
106° in the ion and split into 107° and 108°, a change of less
than 2%. The difference of the N–H distances between the
B3LYP method produces superior IP and stretch frequency in
2
the A state ͑Table II͒, whereas the B3P86 method appears
Ј
to yield better intensity profile.
Theoretical calculations have also been carried out on
HGaNH2 and HGaϩNH2. The ground state of HGaNH2 is
1
2A (C ), and the ground state of HGaϩNH is A (C2v).
Ј
s
2
1
2A and 1A state is only 0.6%. In contrast, ionization causes
The 1A state is 6.96 eV ͑B3LYP͒ or 7.47 eV ͑B3P86͒ higher
Ј
Ј
1
2
2
the Ga–N distance to shrink by 6%. These predictions sug-
gest that the most active mode in the ZEKE spectrum is the
intermolecular gallium–nitrogen stretch, as this mode under-
goes the largest coordinate displacement upon ionization.
The symmetric gallium–nitrogen stretch mode is calculated
than the A state. The A state of the insertion structure is
Ј Ј
predicted to be 2.0 kcal molϪ1 lower by B3LYP, but 0.4
kcal molϪ1 higher by B3P86, than the 2A state of the adduct
Ј
Ga–NH3, in contrast to the Al and In analogues.19,28 The
inserted isomer HAlNH2 is lower in energy than the adduct
to be 277 cmϪ1 ͑B3P86͒ or 266 cmϪ1 ͑B3LYP͒ in the A1
Al–NH3 by about 20 kcal cmϪ1 28
, whereas HInNH2 is higher
1
state, and 202 cmϪ1 ͑B3P86͒ or 174 cmϪ1 ͑B3LYP͒ in the
in energy than In–NH3 by around 10 kcal molϪ1 19
. The for-
2A state. The calculated frequencies in the A state match
mation of HGaNH2 has an energy barrier of 23.6 kcal molϪ1
1
Ј
Ј
well with the experimental values of ␻ϩϭ270 cmϪ1 in the
͑B3LYP͒ or 21.1 kcal molϪ1 ͑B3P86͒. The calculated IP and
s
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J. Chem. Phys., Vol. 115, No. 17, 1 November 2001
Gallium complexes with ammonia
7971
TABLE II. Calculated bond lengths ͑r, Å͒, bond angle ͑Є, degree͒, vibrational frequencies ͑cmϪ1͒, mode
assignments, and ionization potential ͑IP, eV͒ for the gallium-ammonia adduct. H is the symmetry unique H on
Ј
NH3.
B3P86/6-311ϩG(d,p)
B3LYP/6-311ϩG(d,p)
1A /2A
1A /2A
Ј
Ј
1
1
r(Ga–N)
r(N–H)
2.313/2.448
1.021/1.015
/1.014
2.346/2.516
1.022/1.017
/1.016
r(N–H )
Ј
ЄH–N–H
106.2/107.1
/108.2
106.3/107.1
/108.1
ЄH–N–H
Ј
ЄGa–N–H
ЄGa–N–H
112.5/109.5
/114.0
112.5/109.8
/113.8
Ј
N–H stretch, a /a
3445/3489
1288/1120
277/202
3424/3466
1288/1121
266/174
Ј
Ј
Ј
1
NH umbrella, a /a
3
1
Ga–N stretch, a /a
1
N–H stretch, e/a & a
3550/3618 & 3602
1653/1652 & 1543
541/355 & 291
5.57
3522/3587 & 3567
1658/1658 & 1524
536/335 & 285
5.02
Ј
Љ
H–N–H scissors, e/a & a
Ј
Љ
Ga–N–H bend, e/a & a
Ј
Љ
IP
*
spacing between the band origin and band may be due to
insertion barrier exclude HGaNH2 being the carrier of the
ZEKE spectrum, which is also clear from the mismatch of
the simulation ͓Fig. 2͑d͔͒ to the observed spectrum ͓Fig.
2͑a͔͒.
the NH2 rocking (␯ϩ) in Gaϩ –NH2CH3. These assignments
r
are discussed next in combination with the theoretical calcu-
lations.
2. Gallium–monomethylamine
Figure 3͑a͒ presents the ZEKE spectrum of
Ga–NH2CH3. The methylamine complex shows more peaks
than the ammonia species. There are four progressions, a–d,
beginning with the strongest peak at 39 330 cmϪ1. This peak
is assigned to the band origin, which is 805 cmϪ1 lower than
the corresponding band in the Ga–NH3 spectrum. The mem-
bers of progression a are separated by approximate 300
cmϪ1, and those of b by about 125 cmϪ1. Progression c is
formed by one member of a and three members of b,
whereas progression d is a combination of the two intervals
of a and b. In addition, the spectrum exhibits a number of
small peaks, originating from excited levels of the neutral
molecules. These peaks are grouped in e and f. The lowest
energy peak in group e is 93 cmϪ1 below the band origin,
followed by two peaks with the same intervals as those in
progression a. Finally, at 515 cmϪ1 above the origin exists a
*
weak peak labeled . Table I lists the positions and
assignments of all the observed transitions. The Gaϩ –N
stretch, the Gaϩ –N–C bend, and the Ga–N–C bend have
been identified from the spectra. A fit of the ZEKE band
positions in progressions a-d to the formula G(␷ϩ ,␷bϩ)
s
ϭ␻ϩ(␷ϩϩ1/2) ϩ ␻bϩ(␷bϩϩ1/2) ϩ xsϩ(␷ϩϩ1/2)2 ϩ xbϩ(␷
ϩ
s
s
s
b
ϩ1/2)2ϩxϩs xbϩ(␷ϩϩ1/2)(␷ϩb ϩ1/2)]
yielded harmonic
s
frequencies and anharmonicities for the two ion modes:
␻ϩ(Gaϩ –N)ϭ299(1) cmϪ1 and xϩs (Gaϩ –N)ϭϪ1.9(2)
s
cmϪ1, ␻ϩb (Gaϩ –N–C)ϭ124(1) cmϪ1 and xϩb (Gaϩ –N–C)
ϭ0.9(2) cmϪ1, and xsϩxϩb ϭϪ1.1(2) cmϪ1. It is noted that
the Gaϩ –N–C bending has a positive anharmonicity con-
stant. A positive anharmonicity is uncommon for a stretching
mode, but it is not unusual for a bending mode. For example,
the trans-bend of acetylene has an anharmonicity of 3.483
FIG. 3. The ZEKE spectrum of Ga–NH3CH3 ͑a͒ and simulations of the
͑13͒ cmϪ1 29
from the hot bands ͑e, f ͒ is 93 cmϪ1 (␯b). The 515 cmϪ1
.
The frequency of the Ga–N–C bend measured
1A
2A ͑b͒, 1A
2A ͑c͒, and 1A
2A and 1A
2A ͑d͒ transitions of
Љ
Ј
Ј
Ј
Љ
Ј
Ј
Ј
the adduct isomer Ga–NH2CH3 at 80 K.
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7972
J. Chem. Phys., Vol. 115, No. 17, 1 November 2001
Li et al.
TABLE III. Calculated bond lengths ͑r, Å͒, bond angle ͑Є, deg͒, vibrational frequencies ͑cmϪ1͒, mode assign-
ments, and ionization potentials ͑IPs, eV͒ for the gallium-methylamine adduct.
B3P86/6-311ϩG(d,p)
1A /2A /2/A
B3LYP/6-311ϩG(d,p)
1A /2A /2/A
Ј
Љ
Ј
Љ
1
1
r(Ga–N)
R(N–C)
R(N–H)
R(C–H)
2.271/2.395/2.406
1.488/1.468/1.466
1.020/1.016/1.015
1.090/1.096/1.094
1.090/1.091/1.092
111.5/117.2/108.1
110.5/105.2/109.6
109.5/111.3/110.9
105.1/105.8/107.8
115.5/113.5/113.6
109.5/108.9/108.9
108.7/109.1/108.7
109.0/107.2/107.9
3466/3506/3522
3192/3112/3104
3066/3022//3033
1643/1645/1651
1492/1486/1497
1462/1453/1451
1240/1185/1182
1034/933/926
2.305/2.455/2.474
1.498/1.477/1.475
1.021/1.017/1.016
1.091/1.096/1.095
1.090/1.091/1.092
112.7/118.1/110.2
110.0/104.9/108.8
109.4/111.1/110.7
105.1/105.8/107.6
111.3/113.4/113.5
109.4/108.8/108.9
108.8/109.1/108.7
109.1/107.4/108.1
3443/3482/3497
3133/3091/3082
3055/3007/3017
1647/1649/1655
1497/1493/1503
1466/1458/1458
1241//1187/1184
1035/931/924
ЄGa–N–C
ЄGa–N–H
ЄC–N–H
ЄH–N–H
ЄN–C–H
ЄH–C–H
N–H stretch, a
C–H stretch, a
C–H stretch, a
Ј
Ј
Ј
Ј
Ј
NH scissors, a
2
CH scissors, a
2
CH umbrella, a
Ј
3
NH & CH wag, a
Ј
Ј
2
3
NH & CH wag, a
2
3
C–N stretch, a
Ga–N stretch, a
1002/1047/1062
296/223/230
973/1020/1036
284/206/207
Ј
Ј
Ga–N–C bend, a
132/135/113
135/126/108
Ј
N–H stretch, a
C–H stretch, a
3530/3574/3602
3157/3143/3125
1514/1485/1472
1336/1319/1325
995/968/972
3502/3546/3568
3139/3121/3105
1518/1495/1457
1337/1322/1309
995//968/962
Љ
Љ
Љ
CH scissors, a
2
NH & CH twist, a
Љ
Љ
2
3
NH & CH twist, a
2
3
558/450/443
158/143/158
553/443/428
161/143/161
NH rock, a
Љ
Љ
2
CH rock, a
3
IP
/5.48/5.49
/4.93/4.93
Table III presents equilibrium geometries and vibrational
bending (a ) is calculated to be 132 cmϪ1 ͑B3P86͒ or 135
Ј
modes of the neutral and ionic gallium–methylamine adducts
calculated by the B3P86 and B3LYP methods. Ga–NH2CH3
is predicted to have Cs symmetry and two low-lying elec-
tronic states, 2A and 2A . The 2A state is 84 cmϪ1
cmϪ1 ͑B3LYP͒, in good agreement with the measured fre-
quency, 124 cmϪ1, from progression b. The frequency of the
2
Ga–N–C bending mode in the neutral A state is predicted
Љ
to be 113 cmϪ1 ͑B3P86͒ or 108 cmϪ1 ͑B3LYP͒, comparable
to the measured value, 93 cmϪ1, from the hot bands e and f.
The only mode with a predicted frequency similar to the
experimental spacing, 515 cmϪ1, is the NH2 rocking. This
mode is calculated to be 558 cmϪ1 ͑B3P86͒ or 553 cmϪ1
͑B3LYP͒. Because the NH2 rocking motion is antisymmetric
Ј
Љ
Ј
͑B3LYP͒ or 56 cmϪ1 ͑B3P86͒ below the A state. The en-
2
Љ
ergy difference between these two doublets is reduced to 37
cmϪ1 with the CCSD͑T͒ single-point energy calculations at
the B3LYP geometries and to 8 cmϪ1 with CCD geometry
2
optimizations. The HOMO of the A state is a mixture of
Ј
65% of Ga ͑p͒, 12% of N ͑sp͒, 15% of C ͑sp͒, 6% of HC(s),
(a ), one quanta transition would not be allowed under gen-
Љ
eral selection rules.30 However, the fundamental of this mode
is in the vicinity of two quanta of the Gaϩ –N stretching and
four quanta of the Gaϩ –N–C bending. It is, therefore, not
surprising that some kind of interaction may exist among
these vibrations. For example, Fermi resonance would lead
to some vibrational activity for an antisymmetric mode. Nev-
2
and 2% of H (s). The HOMO of the A state contains a
Љ
N
similar amount of the Ga p character. The ground state of
1
1
Gaϩ –NH CH is A in C symmetry and the A state lies
Ј
Ј
2
3
s
4.93 eV ͑B3LYP͒ or 5.48 eV ͑B3P86͒ above the neutral dou-
blets. As in the case of Ga–NH3, the IP from the B3LYP
method is closer to the experimental value, 4.876 eV. The
1
*
HOMO of the A state consists of 44% of Ga ͑sp͒, 33% of
ertheless, because the signal of the peak is very small, such
Ј
N ͑sp͒, 8% of C ͑sp͒, 9% of HC(s), and 6% of HN(s). The
HOMOs in both the neutral and ion are anti-bonding charac-
ters between the metal and ligand orbitals.
interaction should be weak.
1
Figure 3͑b͒ shows the simulation of the A
2A tran-
Ј
Ј
sition from the B3P86 geometries and vibrational modes.
This simulation predicts both the Gaϩ –N stretching and
Gaϩ –N–C bending modes to be active, in consistent with
the above assignments based on vibrational frequencies.
However, the simulation displays weaker Gaϩ –N stretching
The frequency of the Gaϩ –N symmetric stretch (a ) in
Ј
the A state is predicted to be 296 cmϪ1 ͑B3P86͒ or 284
1
Ј
cmϪ1 ͑B3LYP͒, matching well to the experimental value,
299 cmϪ1 from progression a. The Gaϩ –N–C symmetric
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J. Chem. Phys., Vol. 115, No. 17, 1 November 2001
Gallium complexes with ammonia
7973
TABLE IV. Ionization potentials ͑IP, cmϪ1͒, vibrational frequencies ͑cmϪ1͒,
and metal ͑M͒ -nitrogen stretching force constants ͑N/m͒ from the ZEKE
spectra (MϭAl, Ga, In). The values in parentheses are the average of the
Ga–N stretching frequencies in the 2A and 2A states, calculated by the
dispersion force is strengthened because polarizability of
NH2CH3 (␣ϭ4.7 Å3) is larger than that of NH3 (␣
ϭ2.81 Å3).34 In comparison with other group 13 metal-
ammonia complexes, the strength of the metal–nitrogen
binding follows the order AlϾGaϾIn ͑Table IV͒. This trend
is different from that in the M–Rg species ͑MϭAl, Ga, In;
RgϭAr, Kr, Xe͒,35 where the dispersion force is expected to
be predominant. In these metal-molecule complexes, dipole-
induced-dipole and induced-dipole-induced-dipole interac-
tions affect the metal-ligand binding. The induction and dis-
persion forces should increase as the metal atomic
polarizabilities increase down the triad ͑␣Alϭ6.8 Å3, ␣Ga
ϭ8.12 Å3, ␣Inϭ10.2 Å3͒.34 The binding strength in these
complexes is contrary to the polarizability pattern. To explain
the trend observed in these complexes, other factors must be
considered. Using the Mulliken population analysis, we have
found that the orbital overlap between M and N follows the
same trend as the force constants, 0.23 between Al and N,
0.13 between Ga and N, and 0.02 between In and N. For the
ionic species, the metal ions bind the ligands more strongly
than the corresponding neutrals. This is expected because
additional charge-dipole forces in the ionic complexes.
Ј
Љ
B3LYP and B3P86 methods.
a
b
Ga–NH3
Ga–NH2CH3
39 330
Al–NH3
39 746
In–NH3
39 689
IP
40 135
Frequency
␻
␯
ϩ(Mϩ–N)
270
161
299
͑217͒
124
93
339
227
234
141
s
͑M–N͒
s
␻
ϩb (Mϩ–N–C)
␯b(M–N–C)
Force constant
Mϩ–N
0.59
0.21
1.10
͑0.58͒
0.71
0.32
0.48
0.17
M–N
aFrom Ref. 18.
bFrom Ref. 19.
but stronger Gaϩ –N–C bending intensities than those ob-
served in progressions a and b in Fig. 3͑a͒. Because the
2
energy of the A state is essentially the same as that of the
Љ
2A state, a simulation for the A
2A transition has also
1
Ј
Ј
Љ
been performed and is shown in Fig. 3͑c͒. Contrasting the
1A
2A transition, the 1A
2A transition exhibits only a
Љ
IV. CONCLUSIONS
Ј
Ј
Ј
Gaϩ –N stretching progression. The distinct FC activities in
We have obtained the adiabatic IPs and intermolecular
vibrational frequencies for the weakly bound gallium-
ammonia and -monomethylamine complexes. We have found
that the methyl substitution for hydrogen in ammonia en-
hances the binding of the gallium atom and ion with the
nitrogen-containing molecule. This effect can be understood
by considering the dipole, polarization, and orbital interac-
tions in these systems. In comparison with the other group 13
metals we have found that the strength of the gallium–
nitrogen binding follows the order AlϾGaϾIn. In combina-
tion with the electronic structure and FC calculations, we
the two transitions reflect a larger change of the ЄGa–N–C
2
angle in the A state and a greater variation of the Ga–N
Ј
2
distance in the A state upon ionization. The question is
Љ
whether the 1A
2A transition also contributes to the
Ј
Љ
ZEKE spectrum. Figure 3͑d͒ shows a simple addition of the
two simulations in Figs. 3͑b͒ and 3͑c͒, which exhibits a bet-
ter, thought not perfect, intensity match to the experimental
spectrum than the 1A
2A simulation ͓Fig. 3͑b͔͒ alone.
Ј
Ј
Based on this comparison, we assign the ZEKE spectrum to
1
1
both the A
2A and A
2A transitions. The discrep-
Ј
Ј
Ј
Љ
ancy between the experiment ͓Fig. 3͑a͔͒ and the theory ͓Fig.
3͑d͔͒ is assumed to arise from computational errors in the
equilibrium geometries, vibrational coordinates, and the har-
monic model used in simulation. Nevertheless, the overall
agreement between the observed spectrum and the simula-
tion is satisfactory for the purpose of the spectral assignment.
We have also simulated spectra using the B3LYP geometries
and vibrational modes and found that the B3LYP method
yielded simulations similar to those from the B3P86 calcula-
tions.
have identified the 1A1(C3v
)
2A (C ) transition for the
Ј
s
ZEKE spectrum of Ga–NH3, and the 1A (C ) 2A (C )
Ј
Ј
s
s
and 1A (C ) 2A (C ) transitions for the spectrum of
Ј
Љ
s
s
Ga–NH2CH3.
ACKNOWLEDGMENT
This work is supported by the National Science Founda-
tion and the Donors of the Petroleum Research Fund, admin-
istrated by the American Chemical Society. The authors
thank Dennis Clouthier and Richard Judge for the help in
migrating the spectral simulation program from UNIX to Mi-
crosoft Windows operating system and James Watson for
valuable discussions.
C. Gallium–nitrogen binding
The strength of the gallium–nitrogen binding is stronger
in gallium–methylamine than in gallium-ammonia. This is
indicated by the force constants of the metal-ligand stretch-
ing in Table IV. These constants were converted from the
stretch frequencies by considering the complexes as diatomic
molecules.19,31 An enhancement of metal–nitrogen binding
upon a methyl substitution in NH3 was also observed in
sodium32,33 and indium complexes.21 This methyl substituent
effect arises from increased orbital and dispersion interac-
tions. A Mulliken population analysis with the B3LYP
method predicts that the orbital overlap between Ga and N is
0.13 in Ga–NH3 and increases to 0.19 in Ga–NH2CH3. The
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