Reactions of atomic silicon and germanium with ammonia: A matrix-isolation FTIR and theoretical study

Article abstract of DOI:10.1021/jp012928z

The reactions of Si atoms with NH₃ molecules have been studied using matrix-isolation FTIR spectroscopy and density functional theoretical calculations. Silicon atoms reacted with NH₃ to form the SiNH₃ complex spontaneousl

Full text of DOI:10.1021/jp012928z

J. Phys. Chem. A 2002, 106, 3077-3083
3077
Reactions of Atomic Silicon and Germanium with Ammonia: A Matrix-Isolation FTIR and
Theoretical Study
Mohua Chen, Aihua Zheng, Hao Lu, and Mingfei Zhou*
Department of Chemistry and Laser Chemistry Institute, Fudan UniVersity, Shanghai 200433, P. R. China
ReceiVed: July 30, 2001; In Final Form: January 14, 2002
The reactions of Si atoms with NH3 molecules have been studied using matrix-isolation FTIR spectroscopy
and density functional theoretical calculations. Silicon atoms reacted with NH3 to form the SiNH3 complex
spontaneously in solid argon upon annealing. Mercury arc photolysis with a 290-nm long-wavelength pass
filter destroyed the SiNH3 absorptions and produced HSiNH2 absorptions. Full-arc irradiation (250-580 nm)
destroyed the HSiNH2 absorptions and produced SiNH absorptions. The potential energy surface along the
reaction path leading to the observed products has also been calculated. Similar experiments with Ge gave
spectra for the analogous product molecules.
1. Introduction
with NH3 molecules in solid argon. Recent studies in our
laboratory have shown that the combination of matrix-isolation
FTIR and density functional theoretical calculations has been
extremely beneficial in understanding and interpreting reaction
mechanisms.15-17 Although this study seems parallel to that of
NH3 with group 13 metal atoms, we will show that the chemistry
is distinctly different because of the intrinsic difference between
Si and group 13 metals. We will show that ground-state silicon
atoms form a complex with ammonia spontaneously in solid
argon. The complex undergoes photon-induced rearrangement
to form the insertion intermediate HSiNH2, which decomposes
to SiNH upon full-arc mercury lamp irradiation.
Interest in the study of the surface chemistry of silicon has
been enhanced over the past few decades. The silicon surface
is the most commonly used surface in device manufacture
because of its anisotropic etching properties. Nitridation of
silicon surface creates silicon nitride films, which are widely
used in integrated circuits as insulating and passivating layers.
NH3 is an excellent nitriding agent for the production of ultrathin
silicon nitride interfaces which are necessary for the next
generation of very large integrated devices.1 The experimental
and theoretical modeling of the surface reactions at the atomic-
molecular level is particularly helpful in providing better insight
into the mechanism of surface reactions, in which reactive
intermediates might be involved. The interaction of ammonia
with bulk silicon surfaces or silicon clusters has been the subject
of many experimental and theoretical studies.2-4 To our
knowledge, there have been no reports on the reactions of atomic
Si with ammonia molecules.
Particularly relevant to present study is the previous genera-
tion and spectroscopic identification of HSiNH2, H2SiNH, SiNH,
and HSiN by the photon-induced decomposition of precursor
molecules in solid matrixes.18-20 The geometries, relative
stabilities, and reactivities of these silicon-nitrogen containing
species have also been the subject of several theoretical
studies.21-23
Matrix-isolation spectroscopic methods have been widely
used to investigate the reactions of transition metal atoms, as
well as main group atoms, with small molecules such as H2,5
O2,6 H2O,7 and NH3.8 In solid matrixes, Fe, Ni, and Cu atoms
formed adduct with one or two NH3 molecules,9-11 and UV
irradiation induced the insertion of the metal into a N-H bond
with the formation of the amido derivatives HMNH2 (M ) Fe,
Ni, and Cu), HMNH2NH3 (M ) Fe or Ni), and H + MNH2 (M
) Cu). The reactions between group IIIA atoms and ammonia
in solid matrixes have been studied by several groups. An EPR
study revealed the formation of Al(NH3)2 and Al(NH3)4 ad-
ducts.12 Andrews and co-workers reported that electronically
excited Al atoms reacted with ammonia to form the HAlNH2
and AlNH2 molecules.13 Recently, the thermal and photo-
chemical reactions of the group IIIA metals Al, Ga, and In with
ammonia have been investigated by Himmel et al., who
characterized the MNH3, HMNH2, MH2, and H2MNH2 species
that were generated.14
2. Experimental and Theoretical Methods
The technique used for pulsed laser ablation and matrix-
isolation infrared spectroscopic investigation has been described
in detail previously.24 The 1064-nm fundamental of a Nd:YAG
laser (Spectra Physics, DCR 2, 20-Hz repetition rate and 8-ns
pulse width) was focused onto a rotating silicon or germanium
target through a hole in a CsI window. Typically, 5-10 mJ/
pulse of laser power was used. The ablated Si or Ge atoms were
codeposited with molecular NH3 in excess argon onto an 11-K
CsI window at a rate of 2-4 mmol/h. The CsI window was
mounted on a copper holder at the cold end of a cryostat (Air
Products Displex DE 202), and the temperature was maintained
by a closed-cycle helium refrigerator (Air Products Displex
1R02W). FTIR spectra were recorded on a Bruker IFS 113v
Fourier transform infrared spectrometer equipped with a DTGS
detector with a resolution of 0.5 cm-1. Isotopic 15NH3 was
prepared by the thermal dissociation of (15NH4)2SO4 (98%).
Matrix samples were annealed at different temperatures and
subjected to photolyses at different wavelengths using a high-
pressure mercury lamp (250-W, without globe) and glass filters.
In this paper, we report a combined matrix-isolation FTIR
spectroscopic and theoretical study of the reactions of Si atoms
* Corresponding author. E-mail: mfzhou@fudan.edu.cn. Fax: +86-21-
65643532.
10.1021/jp012928z CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/07/2002

3078 J. Phys. Chem. A, Vol. 106, No. 13, 2002
Chen et al.
TABLE 1: Infrared Absorptions (cm-1) following the Reaction of Si Atoms with NH3 Molecules in an Argon Matrix
Si + 14NH3
Si + 15NH3
annealinga
λ > 290 nma
λ > 250 nma
assignmentb
3570.8
3387.1
1927.0
1599.3
1196.9
1185.5
938.6
853.8
562.8
514.9
510.0
3561.3
3378.7
1926.7
1596.0
1173.2
1180.0
934.3
838.0
559.2
512.3
507.5
v
SiNH, ν(N-H)
SiNH3, νasym(N-H)
HSiNH2, ν(H-Si)
SiNH3, δasym(NH3)
SiNH, ν(Si-N)
SiNH3, δsym(NH3)
HSiNH2, â(NH2)
HSiNH2, ν(Si-N)
HSiNH2, ω(NH2)
SiNH, δ(SiNH)
SiNH, site
v
v
v
V
v
V
V
v
V
v
v
v
V
V
V
v
v
a v, increase; V, decrease. b ν, stretching mode; δ, bending; â, wagging; ω, out-of-plane rocking.
Figure 2. Infrared spectra in the 1980-1575 and 1205-485 cm-1
regions following the codeposition of laser-ablated Si atoms with 0.1%
14NH3 + 0.2% 15NH3 in argon: (a) as-deposited at 11 K, (b) after 25-K
annealing, (c) after 20 min of λ > 290 nm photolysis, and (d) after 20
min of λ > 250 nm photolysis.
Figure 1. Infrared spectra in the 1975-1580 and 1220-480 cm-1
regions following the codeposition of laser-ablated Si atoms with 0.2%
NH3 in argon: (a) as-deposited at 11 K, (b) after 25-K annealing, (c)
after 20 min of λ > 290 nm photolysis, and (d) after 20 min of λ >
250 nm photolysis.
853.8, and 562.8 cm-1 absorptions disappeared with the
simultaneous growth of a set of bands at 3570.8, 1196.9, and
514.9 cm-1 (with a site band at 510.0 cm-1).
N-15 isotopic substitution was employed for band identifica-
tion, with the results included in Table 1. All of the new product
absorptions underwent substantial shifts in the experiment
employing 15NH3. Figure 2 shows the mixed 14NH3 + 15NH3
spectra in selected regions.
Ge + NH3. Similar experiments were also performed for Ge
+ NH3 reaction. The IR absorption spectra in selected regions
are presented in Figure 3, with the absorptions listed in Table
2. In addition to the absorptions due to NH3, (NH3)x, H2O, and
NH3‚H2O, new features at 1578.4 and 1128.7 cm-1 appeared
upon sample deposition and markedly increased upon annealing.
These two bands disappeared upon high-pressure mercury lamp
photolysis with a 290-nm long-wavelength pass filter, while a
set of new absorptions at 1793.3, 863.2, 692.8, and 474.0 cm-1
was developed. Subsequent full-arc photolysis almost destroyed
these bands. The spectra for a 15NH3/Ar sample are shown in
Figure 4. All of the new product absorptions exhibited isotopic
shifts, as listed in Table 2.
Calculation Results. B3LYP/6-311++G(d,p) calculations
were done on three MNH3 isomers (M ) Si, Ge), namely, the
MNH3 complex and the inserted HMNH2 and H2MNH mol-
ecules in both singlet and triplet states. The optimized geometric
parameters are shown in Figure 5 (the parameters of Ge-
containing species are given in parentheses), and the vibrational
frequencies and intensities are listed in Tables 3 and 4. The
Density functional calculations were performed using the
Gaussian 98 program.25 The three-parameter hybrid functional
of Becke with additional correlation corrections due to Lee,
Yang, and Parr (B3LYP)26,27 and the 6-311++G(d,p) basis set
were used.28,29 The geometries of products were fully optimized
and vibrational frequencies were calculated with analytic second
derivatives. Transition state optimizations were done with the
synchronous transit-guided quasi-Newton (STQN) method,30
followed by vibrational frequency calculations showing the
obtained structures to be true saddle points.
3. Results
FTIR spectra of the Si + NH3/Ar and Ge + NH3/Ar systems
and DFT calculation results are presented in this section.
Si + NH3/Ar. Representative spectra in selected regions from
the co-condensation of laser-ablated Si atoms with 0.2% NH3
in argon are shown in Figure 1, and the infrared absorptions
are summarized in Table 1. One-hour sample deposition at 11
K resulted in strong NH3 and weak (NH3)n (n > 1), H2O, and
NH3‚H2O absorptions.31,32 Three new absorptions at 3387.1,
1599.3, and 1185.5 cm-1 were also observed after sample
deposition and increased together markedly upon annealing.
Upon subsequent high-pressure mercury lamp photolysis with
a 290-nm long-wavelength pass filter, the absorptions at 3387.1,
1599.3, and 1185.5 cm-1 were destroyed, and new absorptions
at 1927.0, 1557.8, 938.6, 853.8, and 562.8 cm-1 were produced.
Upon further full-arc photolysis, the 1927.0, 1557.8, 938.6,

Reactions of Atomic Si and Ge with Ammonia
J. Phys. Chem. A, Vol. 106, No. 13, 2002 3079
TABLE 2: Infrared Absorptions (cm-1) following the Reaction of Ge Atoms and NH3 Molecules in an Argon Matrix
Ge + 14NH3
Ge + 15NH3
annealinga
λ > 290 nma
λ > 250 nma
assignmentb
1793.3
1787.9
1578.4
1128.7
1125.1
863.2
692.8
474.0
460.3
1793.2
1787.7
v
v
V
V
V
v
v
v
v
V
V
HGeNH2,ν(H-Ge)
HGeNH2, site
v
v
v
GeNH3, δasym(NH3)
GeNH3, δsym(NH3)
GeNH3, site
1123.2
1119.8
859.3
678.4
471.3
457.3
V
V
V
V
HGeNH2, â(NH2)
HGeNH2, ν(Ge-N)
HGeNH2, ω(NH2)
HGeNH2, site
a v, increase; V, decrease. b ν, stretching mode; δ, bending; â, wagging; ω, out-of-plane rocking.
Figure 3. Infrared spectra in the 1880-1500, 1180-650 and 500-
450 cm-1 regions following the codeposition of laser-ablated Ge atoms
with 0.2% NH3 in argon: (a) as-deposited at 11 K, (b) after 25-K
annealing, (c) after 20 min of λ > 290 nm photolysis, and (d) after 20
min of λ > 250 nm photolysis.
Figure 5. Calculated geometric parameters (bond lengths in angstroms,
bond angles in degrees) of the MNH3, HMNH2, H2MNH, MNH, and
NMH (M ) Si and Ge) molecules. Values for Ge compounds are listed
in parentheses.
4. Discussion
The product absorptions were assigned by consideration of
the frequencies and isotopic shifts of the observed bands and
by comparisons with DFT frequency calculations. We then
focused on the reaction mechanisms and the potential energy
surfaces of the corresponding M + NH3 reactions (M denotes
Si or Ge).
SiNH3. According to the annealing and photolysis behavior
at different temperatures, the bands at 3387.1, 1599.3, and
1185.5 cm-1 belong to different vibrational modes of a common
species. In the Si + 15NH3 experiment, these three bands shifted
to 3378.7, 1596.0, and 1180.0 cm-1 and exhibited 14N/15N ratios
of 1.0024, 1.0019, and 1.0047, respectively. The band positions,
relative intensities, and isotopic ratios of these three bands are
very close to those of the antisymmetric N-H stretching,
antisymmetric, and symmetric NH3 deformation modes of a free
NH3 molecule in an argon matrix (for NH3, 3447.3 cm-1/1.0023,
1638.7 cm-1/1.0018, 974.7 cm-1/1.0043), suggesting that these
absorptions are due to one or more NH3 molecules perturbed
by Si atoms. In the mixed 14NH3 + 15NH3 experiment, only
pure isotopic counterparts were observed for each mode,
indicating that only one NH3 unit is involved in this molecule.
Judging from the experimental conditions, it seems unlikely that
more than one Si atom is involved. We note that, under essen-
tially the same experimental conditions, laser-ablated Si reacted
with O2 to form SiO and SiO2, and no di- or multi-silicon-
containing species were produced upon sample deposition or
even upon 30-K annealing.33 Accordingly, we assigned the
3387.1, 1599.3, and 1185.5 cm-1 bands to the SiNH3 molecule.
Figure 4. Infrared spectra in the 1850-1545, 1150-665 and 500-
450 cm-1 regions following the codeposition of laser-ablated Ge atoms
with 0.2% 15NH3 in argon: (a) as-deposited at 11 K, (b) after 25-K
annealing, (c) after 20 min of λ > 290 nm photolysis, and (d) after 20
min of λ > 250 nm photolysis.
SiNH3 and GeNH3 complexes were predicted to have 3A1
ground states with C3V symmetry. Both the HMNH2 and
H2MNH (M ) Si and Ge) isomers were calculated to have 1A′
ground states with planar structures.
Calculations were also performed on the MNH and HMN
(M ) Si, Ge) isomers. The optimized structures are also shown
in Figure 5, with the vibrational frequencies and intensities
summarized in Tables 3 and 4.

3080 J. Phys. Chem. A, Vol. 106, No. 13, 2002
Chen et al.
TABLE 3: Calculated Vibrational Frequencies (cm-1) and
Intensities (km/mol) of the Products and Transition States in
the Si + NH3 Reaction
tively, which required scaling factors of 0.95, 0.97, and 0.97 to
fit the observed values. As listed in Table 3, the calculated
isotopic frequency shifts are also in good agreement with the
experimental values.
species
frequency (intensity, mode)
GeNH3. Similar bands at 1578.4 and 1128.7 cm-1 in the Ge
+ NH3/Ar experiments were assigned, respectively, to the
antisymmetric and symmetric NH3 deformation modes of the
GeNH3 molecule. DFT calculations on GeNH3 predicted that
the molecule has a C3V triplet ground state. The calculated
antisymmetric and symmetric NH3 deformation frequencies,
1655.1 and 1165.8 cm-1, are in quite good agreement with the
experimental values. The N-H stretching modes were not
observed in our experiments and were possibly masked by other
absorptions or too weak to be observed.
3563.8 (67 × 2, e); 3455.2 (13, a1); 1651.8 (33 × 2, e);
1221.6 (94, a1); 544.2 (7 × 2, e); 328.6 (37, a1)
3553.2 (67 × 2); 3452.3 (12); 1648.8 (33 × 2);
1215.8 (91); 543.0 (7 × 2); 322.8 (36)
SiNH3 (3A1)
Si15NH3
3644.9 (36, a′); 3552.4 (16, a′); 2001.6 (376, a′);
HSiNH2 (1A′) 1602.7 (41, a′); 969.2 (118, a′); 844.2 (77, a′);
739.8 (1, a′′); 703.6 (2, a′); 601.9 (251, a′′)
3634.2 (36); 3547.4 (15); 2001.6 (376); 1597.1 (38);
965.0 (118); 828.6 (74); 739.7 (1); 702.8 (2); 597.9 (248)
3585.3 (23, a′); 2290.4 (74, a′); 2197.5 (115, a′);
H2SiNH (1A′) 1121.2 (50, a′); 1000.1 (55, a′); 769.0 (42, a′);
744.8 (99, a′); 605.1 (60, a′′); 570.6 (76, a′)
HSi15NH2
SiNH (1Σ+)
Si15NH
3743.0 (96, σ); 1230.0 (69, σ); 544.4 (219 × 2, π)
3733.0 (92); 1205.3 (67); 541.4 (216 × 2)
3610.8 (4 × 2, e); 3483.7 (2, a1); 1668.4 (28 × 2, e);
1002.1 (214, a1)
3591.7 (44, b); 3511.7 (7, a1); 1585.6 (30, a1);
840.6 (88, a1); 742.8 (14, b2); 591.5 (252, b1)
3589.6 (43); 3477.8 (9); 1603.3 (61); 1520.9 (12);
903.9 (0); 690.7 (191); 643.6 (12); 501.6 (109);
1365.3i (421)
3559.2 (122); 3315.8 (80); 1735.7(123); 1629.3 (29);
1360.5 (1); 838.8 (167); 624.9 (149); 491.6 (100);
1510.0i (1798)
3603.4 (33); 1900.4 (125); 1787.0 (119); 1109.6 (1);
992.4 (123); 890.5 (334); 674.2 (111); 585.5 (165);
1805.7i (748)
HSiNH2. The bands at 1927.0, 1557.8, 938.6, 853.8, and
562.8 cm-1 appeared together upon broad-band photolysis
(290-580 nm) with the simultaneous disappearance of the
SiNH3 absorptions. These bands were assigned to the HSiNH2
molecule. In the 15NH3 experiment, these five bands shifted to
1926.7, 1552.6, 934.3, 838.0, and 559.2 cm-1, respectively. The
strongest absorption at 1927.0 cm-1 is close to the Si-H
NSiH (1Σ+)
SiNH2 (2B2)
TS1
stretching vibrations of HSiOCH3 (1925 cm-1 34
(1882 cm-1 35 and undergoes little shift upon 15N substitution,
)
and HSiOH
)
TS2
TS3
TS4
indicating that this band might be due to a terminal Si-H
stretching vibration. The 853.8 cm-1 band exhibited 15.8 cm-1
15N shift and is due to a Si-N stretching mode. The bands at
1557.8, 938.6, and 562.8 cm-1 are due to NH2 scissoring,
wagging, and out-of-plane rocking vibrations, respectively.
3396.8(2); 2025.7(233); 1929.0(77); 1035.3(158);
925.4(33); 848.2(79); 729.1(125); 509.1(13);
1403.0i (522)
The HSiNH2 molecule has been previously prepared by 254-
nm light irradiation of silyl azide (H3SiN3) in solid Ar and
identified spectroscopically by Maier and co-workers.19,20 Six
vibrational fundamentals including symmetric and antisymmetric
N-H stretching, Si-H stretching, NH2 scissoring, Si-N
stretching, and NH2 out-of-plane rocking have been assigned,
with each mode exhibiting several site splittings. The band
positions of the Si-H and Si-N stretching, NH2 scissoring,
and rocking modes in our experiments are lower than the most
intense site absorptions reported by Maier et al.19 This suggests
that most of the HSiNH2 molecules trapped in the argon matrix
by Maier and co-workers might be due to HSiNH2-X com-
plexes (X denotes any other species in the system, such as the
eliminated N2). The NH2 wagging vibration at 938.6 cm-1
reported here was not observed by Maier et al. This mode
probably was masked by absorptions of the H3SiN3 precursor
in their experiments. The two N-H stretching modes were too
weak to be observed in our experiments.
In good agreement with previous theoretical calculations,21
the present DFT calculations at the B3LYP/6-311++G(d,p)
level predict that the HSiNH2 molecule has a singlet ground
state with planar Cs symmetry and is the global minimum on
the SiNH3 potential energy surface. As listed in Table 3, the
six observed vibrations were calculated at 2001.6, 1602.7, 969.2,
844.2, and 601.9 cm-1. Both the vibrational frequencies and
the isotopic frequency shifts matched the observed values very
well.
TABLE 4: Calculated Vibrational Frequencies (cm-1) and
Intensities (km/mol) of the Products and Transition States in
the Ge + NH3 Reaction
species
GeNH3 (3A1)
Ge15NH3
frequency (intensity, mode)
3587.9 (56 × 2, e); 3471.8 (15, a1); 1655.1 (29 × 2, e);
1165.8 (108, a1); 486.8 (10 × 2, e); 280.0 (22, a1)
3577.9 (56 × 2); 3469.7 (14); 1651.9 (29 × 2);
1159.7 (104); 485.2 (10 × 2); 273.6 (21)
3662.1 (31, a′); 3555.9 (15, a′); 1853.9 (449, a′);
HGeNH2 (1A′) 1578.2 (24, a′); 885.8 (73, a′); 688.5 (2, a′′);
681.2 (51, a′); 643.5 (32, a′); 505.0 (245, a′′)
3651.1 (31); 3551.1 (14); 1853.9 (449); 1572.8 (22);
881.7 (74); 688.4 (2); 669.7 (32); 638.5 (47); 501.7 (243)
HGe15NH2
GeNH (1Σ+)
3697.6 (85, σ); 989.1 (78, σ); 424.1 (232 × 2, π)
3474.6 (9, a′); 2185.1 (71, a′); 2085.0 (113, a′);
H2GeNH (1A′) 950.9 (52, a′); 868.5 (55, a′); 865.5 (35, a′);
769.6 (47, a′′); 532.5 (44, a′); 503.3 (13, a′′)
NGeH (1Σ+)
GeNH2 (2B2)
2113.4 (31, σ); 1037.3 (0, σ); 273.3 (10 × 2, π)
3621.7 (40, b2); 3524.4 (9, a1); 1565.9 (18, a1);
702.3 (16, b2); 662.9 (82, a1); 498.7 (249, b1)
3605.0 (36); 3488.5 (13); 1532.3 (55); 1485.6 (68);
845.9 (0); 609.3 (141); 575.3 (18); 392.5 (83);
1150.0i (143)
3588.1 (89); 3386.2 (23); 1646.4 (69); 1481.5 (46);
1242.5 (12); 719.1 (126); 515.2 (154); 409.8 (100);
1579.3i (1876)
3536.6 (14); 1868.3 (121); 1697.6 (115); 1102.9 (1);
868.7 (235); 789.2 (57); 752.0 (137); 511.4 (142);
1821.3i (737)
3367.1 (1); 1934.0 (232); 1801.2 (40); 1009.5 (128);
857.9 (34); 726.9 (40); 703.6 (80); 520.0 (10);
1265.0i (308)
TS1
TS2
TS3
TS4
HGeNH2. Similar absorptions at 1793.3, 863.2, 692.8, and
474.0 cm-1 can be assigned to the HGeNH2 molecule. The
1793.3 cm-1 absorption exhibited no obvious N-15 shift and is
due to a Ge-H stretching vibration. The 692.8 cm-1 band
shifted to 678.4 cm-1 with 15NH3 and is due to a Ge-N
stretching vibration. The bands at 863.2 and 474.0 cm-1
exhibited small N-15 shifts and are NH2 wagging and out-of-
plane rocking vibrations, respectively. The HGeNH2 molecule
was predicted to have a singlet ground state with planar Cs
Density functional calculations predicted the SiNH3 molecule
to have a 3A1 ground state with C3V symmetry, which correlates
3
to the Si P ground state. The antisymmetric N-H stretching
and antisymmetric and symmetric NH3 deformation modes were
calculated to lie at 3563.8, 1651.8, and 1221.6 cm-1, respec-

Reactions of Atomic Si and Ge with Ammonia
J. Phys. Chem. A, Vol. 106, No. 13, 2002 3081
symmetry analogous to HSiNH2. As listed in Table 4, the
calculated vibrational frequencies and isotopic frequency shifts
of the Ge-H stretching, NH2 wagging, Ge-N stretching, and
NH2 out-of-plane rocking vibrations are in good agreement with
the observed values.
SiNH. The absorptions at 3570.8, 1196.9, and 514.9 cm-1
that appeared together upon full-arc photolysis are assigned to
the SiNH molecule. The 1196.9 cm-1 band exhibited a 23.7
cm-1 N-15 shift and is due to a Si-N stretching vibration. The
3570.8 cm-1 band shifted to 3561.3 cm-1 with 15NH3 and is a
N-H stretching vibration. The 514.9 cm-1 band underwent a
2.6 cm-1 N-15 shift and is attributed to the SiNH bending
vibration. The infrared spectrum of SiNH in solid argon matrix
has been previously reported by Ogilivie and Cradock.18 The
three vibrational fundamentals were observed at 3583, 1198,
and 523 cm-1, in reasonable agreement with our values. Recent
high-resolution infrared emission spectra of SiN-H stretching
Figure 6. Potential energy surface along the Si + NH3 reaction path
calculated at the B3LYP/6-311++G(d,p) level. Energies are given in
kcal/mol and are relative to the reactants Si(3P) + NH3(1A1).
in the gas phase revealed a value of 3588.4 cm-1 36
Our DFT
.
calculations predicted that the SiNH molecule has a 1Σ+ ground
state, in agreement with previous theoretical calculations.22,23
As listed in Table 3, the calculated vibrational frequencies and
relative intensities matched the experimental values.
Reaction Mechanism. Co-condensation of laser-ablated Si
or Ge atoms with ammonia in excess argon at 11 K formed
SiNH3 and GeNH3 complexes. The absorptions of these
complexes increased markedly upon annealing, indicating that
ground-state Si or Ge atoms react with NH3 to form the
complexes with no activation energy
Si(3P) + NH3 f SiNH3(3A1) ∆E ) -19.6 kcal/mol (1)
Ge(3P) + NH3 f GeNH3(3A1) ∆E ) -17.4 kcal/mol (2)
Figure 7. Potential energy surface along the Ge + NH3 reaction path
calculated at the B3LYP/6-311++G(d,p) level. Energies are given in
kcal/mol and are relative to the reactants Ge(3P) + NH3(1A1).
The SiNH3 and GeNH3 absorptions disappeared upon photolysis
at λ > 290 nm with the emergence of the HSiNH2 and HGeNH2
absorptions, suggesting that the HSiNH2 and HGeNH2 molecules
are generated from SiNH3 and GeNH3 via reactions 3 and 4,
which were predicted to be exothermic
SiNH3(3A1) f HSiNH2(1A′) ∆E ) -46.3 kcal/mol (3)
GeNH3(3A1) f HGeNH2(1A′) ∆E ) -29.8 kcal/mol (4)
Subsequent full-arc photolysis destroyed the HSiNH2 absorp-
tions and produced the SiNH absorptions. The SiNH molecules
are most likely produced by H2 elimination from the HSiNH2
molecules via reaction 5. In the Ge experiments, the HGeNH2
absorptions were also destroyed upon full-arc photolysis, but
no obvious absorptions in the 4000-400 cm-1 region were
produced. Analogously to Si, the formation of GeNH via H2
elimination, reaction 6, was predicted to be slightly endothermic.
DFT calculations for the 1Σ+ ground-state GeNH provided
frequencies at 3697.6, 989.1, and 424.1 cm-1 with relative
intensities of 85:78:464. The most intense bending vibration is
probably under 400 cm-1, which is out of our detection range.
The N-H stretching and Ge-N stretching vibrations are much
weaker and could easily be masked by strong NH3 or (NH3)n
absorptions.
Figure 8. Calculated geometric parameters (bond lengths in angstroms,
bond angles in degrees) of the transition states. Values for the Ge
reaction system are listed in parentheses.
To obtain a better understanding of the reaction mechanism,
potential energy surfaces along the reaction paths were calcu-
lated, and the results are shown in Figures 6 and 7, with the
optimized transition state structures shown in Figure 8. On both
the singlet and triplet potential energy surfaces of the Si + NH3
reaction, the first step is the formation of the SiNH3 complex;
this adduction reaction is exothermic and barrier-free. From the
SiNH3 complex, one H atom is passed from N to Si, leading to
the HSiNH2 intermediate through transition states TS1 and TS2.
As the SiNH3 complex has a triplet ground state whereas the
HSiNH2(1A′) f SiNH(1Σ+) + H2
∆E ) +13.7 kcal/mol (5)
HGeNH2(1A′) f GeNH(1Σ+) + H2
∆E ) +19.6 kcal/mol (6)

3082 J. Phys. Chem. A, Vol. 106, No. 13, 2002
Chen et al.
HSiNH2 has a singlet ground state, there is spin crossing from
triplet SiNH3 to singlet HSiNH2. The transition state TS1 lies
about 6.2 kcal/mol above the energy of the ground-state
reactants, and the energy barrier was estimated to be 25.8 kcal/
mol.
From singlet HSiNH2, two reaction paths are possible. One
path is H2 elimination to form SiNH through transition state
TS3. Another path is one-hydrogen-atom transfer from nitrogen
to the Si center to form H2SiNH through transition state 4 (TS4).
TS3 lies about 9.2 kcal/mol lower in energy than TS4, indicating
that the energy barrier for the formation of SiNH is 9.2 kcal/
mol lower than that for the formation of H2SiNH. The
isomerization reaction from HSiNH2 to H2SiNH has been
previously studied by Truong and Gordon with ab initio
methods.21 The energy barrier of this isomerization reaction was
predicted to be about 78 kcal/mol, 12.6 kcal/mol higher than
our DFT value.
Figure 9. Potential energy surface following the reaction paths from
HAlNH2 leading to AlNH2 + H and AlNH + H2. Energies are given
in kcal/mol and are relative to 2A′ HAlNH2 (HAlNH2, planar Cs
symmetry, H-Al ) 1.604 Å, Al-N ) 1.794 Å, N-H ) 1.010/1.013
Å, HAlN ) 115.7°, AlNH ) 125.6/124.5°; AlNH2, planar C2V
symmetry, Al-N ) 1.820 Å, N-H ) 1.015 Å, AlNH ) 125.8°;
AlNH, bent Cs symmetry, Al-N ) 1.761 Å, N-H ) 1.019 Å, AlNH
) 134.7°).
Maier and co-workers suggested that HSiNH2 might not be
directly transformed into SiNH.20 They assumed that the reaction
proceeded via a H2SiNH intermediate, which has been observed
at 2253, 2175, and 1097 cm-1 in solid argon.20 No H2SiNH
absorptions were observed in our experiments. The present DFT
calculations predict that the H2SiNH molecule has a 1A′ ground
state and is about 16.1 kcal/mol higher in energy than the
HSiNH2 molecule. The calculated potential energy surface also
suggests that H2SiNH is not the intermediate along the reaction
path from HSiNH2 to SiNH + H2.
The Si + NH3 reaction is quite similar to the Si + H2O
reaction reported previously.35 Initially, silicon atoms interact
with water to form a SiOH2 adduct, which rearranges spontane-
ously to give HSiOH. When strongly irradiated, the HSiOH
intermediate undergoes photolytic decomposition to give silicon
monoxide, rather than forming H2SiO.37
the endothermicity for this H2-elimination reaction is only
slightly higher than that of the HAlNH2 f AlNH2 + H reaction,
the H2 elimination reaction proceeds via a transition state with
an energy barrier of 53.7 kcal/mol. Apparently, the formation
of AlNH2 is energetically favored over the formation of AlNH.
The situation was reversed in the case of Si. As shown in Figure
6, the SiNH2 molecule was predicted to have a 2B2 ground state,
and the formation of H + SiNH2 from singlet ground-state
HSiNH2 is a spin-forbidden process that is energetically
unfavorable. No evidence was found for the SiNH2 molecule
in our experiments.
As shown in Figure 7, the potential energy surface for the
Ge + NH3 reaction is quite similar to that of the Si + NH3
reaction. Spin crossing also occurs from the triplet ground-state
GeNH3 to the singlet ground-state HGeNH2. The transition state
TS1 lies 15.2 kcal/mol higher in energy than the separate
reactants, and the energy barrier for this reaction is predicted
to be about 32.6 kcal/mol. Two reaction paths starting from
singlet HGeNH2 are also possible: one path is the formation
of H2GeNH, and the other is the formation of GeNH + H2.
The energy barrier of the reaction path for the formation of
GeNH is 17.1 kcal/mol lower than that of the reaction path for
the formation of H2GeNH.
The photochemical reactions of HMNH2 (M denotes main
group metal atoms including Al, Ga, In, Si, and Ge) are also
quite different from those of early transition metal atoms. Recent
investigations in our laboratory showed that, for Ti and V group
metals, the major photochemical reaction channel of the HMNH2
molecule is the formation of H2MNH.38
5. Conclusions
The reactions between Si and Ge atoms and NH3 molecules
were investigated using the matrix-isolation technique. Various
reaction intermediates and products were produced and trapped
via thermal and photochemical reactions in solid argon and
characterized by infrared spectra and density functional theoreti-
cal calculations. Silicon and germanium atoms reacted with NH3
to form the SiNH3 and GeNH3 complexes spontaneously upon
annealing. Mercury arc photolysis with a 290-nm long-
wavelength pass filter induced isomerization of the complexes
to form the inserted HSiNH2 and HGeNH2 molecules. The
HSiNH2 molecule further decomposed to SiNH + H2 upon full-
arc irradiation (250-580 nm). The potential energy surfaces
along the reaction path leading to the observed products were
calculated to assist in interpreting the reaction mechanism.
The photochemical reaction of HSiNH2 is distinctly different
from those of group 13 metals. Upon exposure to broad-band
UV-visible light, the HMNH2 molecules (M ) Al, Ga, and
In) prefer to lose one H atom to form the monovalent MNH2
molecule, but H2 elimination dominates the HSiNH2 reaction.
The chemistries of HMNH2 (M ) Al, Ga, and In) and HSiNH2
differ so greatly because Si has two valence p electrons whereas
group 13 metals has only one.
The photochemical reactions of HSiNH2 and HGeNH2 are
distinctly different with those of group 13 metals. The reactions
of group 13 metal atoms with NH3 were reported quite
recently,14 and the HMNH2 molecules (M ) Al, Ga, and In)
were found to prefer to decompose to form the monovalent
MNH2 molecule upon exposure to broad-band UV-visible light.
According to calculations, the HMNH2 molecules (M ) Al,
Ga, and In) have doublet ground states, whereas the MNH2
molecules have singlet ground states. Thus, the HMNH2 f H
+ MNH2 reaction conserves spin. As discussed by the authors,
the hydrogen atom expelled in this reaction is able to escape
from the matrix cage because of its small size. For comparison,
we calculated part of the potential energy surface of the Al +
NH3 reaction at the B3LYP/6-311++G(d,p) level, and the
results are shown in Figure 9. From doublet HAlNH2, the loss
of one H atom to form AlNH2 proceeds without a transition
state. This process was predicted to be endothermic by about
40.9 kcal/mol. The HAlNH2 f AlNH + H2 reaction was
calculated to be endothermic by about 44.0 kcal/mol. Although

Reactions of Atomic Si and Ge with Ammonia
J. Phys. Chem. A, Vol. 106, No. 13, 2002 3083
(18) Ogilvie, J. F.; Cradock, S. J. Chem. Soc., Chem. Commun. 1966,
Acknowledgment. We acknowledge support for this re-
search from NSFC (Grant 20003003 and 20125033) and the
Chinese NKBRSF.
364.
(19) Maier, G.; Glatthaar, J.; Reisenauer, H. P. Chem. Ber. 1989, 122,
2403.
(20) Maier, G.; Glatthaar, J. Angew. Chem., Int. Ed. Engl. 1994, 33,
473.
References and Notes
(21) Truong, T. N.; Gordon, M. S. J. Am. Chem. Soc. 1986, 108, 1775.
(22) Melius, C. F.; Ho, P. J. Phys. Chem. 1991, 95, 1410.
(23) Jursic, B. S. THEOCHEM 1999, 460, 11. Kwon, O.; Kwon, Y.
THEOCHEM 1999, 460, 213.
(24) Chen, M. H.; Wang, X. F.; Zhang, L. N.; Yu, M.; Qin, Q. Z. Chem.
Phys. 1999, 242, 81.
(25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
98, revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998.
(26) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(1) See, for example: Fattal, E.; Radeke, M. R.; Reynolds, G.; Carter,
E. A. J. Phys. Chem. B 1997, 101, 8658 and references therein.
(2) Colaianni, M. L.; Chen, P. J.; Yates, J. T., Jr. J. Chem. Phys. 1992,
96, 7826. Stober, J.; Eisenhut, R.; Rangelor, G.; Fauster, T. Surf. Sci. 1994,
321, 111.
(3) Bower, J. E.; Jarrold, M. F. J. Chem. Phys. 1992, 97, 8312.
Patterson, C. H.; Messmer, R. P. Phys. ReV. B 1990, 42, 7530.
(4) Ratcliff, L.; Holme, T. J. Phys. Chem. A 1998, 102, 9531.
(5) See, for example: Fredin, L.; Hauge, R. H.; Kafafi, Z. H.; Margrave,
J. L. J. Chem. Phys. 1985, 82, 3542. Chertihin, G. V.; Andrews, L. J. Am.
Chem. Soc. 1994, 116, 8322. Pullumbi, P.; Bouteiller, Y.; Manceron, L.;
Mijoule, C. Chem. Phys. 1994, 185, 25.
(6) See, for example: Chertihin, G. V.; Saffel, W.; Yustein, J. T.;
Andrews, L.; Neurock, M.; Ricca, A.; Bauschlicher, C. W., Jr. J. Phys.
Chem. 1996, 100, 5261. Chertihin, G. V.; Andrews, L. J. Phys. Chem. 1995,
99, 6356.
(7) See, for example: Kauffman, J. W.; Hauge, R. H.; Margrave, J. L.
J. Phys. Chem. 1985, 89, 3541; 1985, 89, 3547.
(8) See, for example: Suzer, S.; Andrews, L. J. Am. Chem. Soc. 1987,
109, 300. Thompson, C. A.; Andrews, L.; Martin, J. M. L.; El-Yazal, J. J.
Phys. Chem. 1995, 99, 13839.
(9) Kauffman, J. W.; Hauge, R. H.; Margrave, J. L. High Temp. Sci.
1984, 17, 237. Szczepanski, J.; Szczesniak, M.; Vala, M. Chem. Phys. Lett.
1989, 162, 123.
(27) Lee, C.; Yang, E.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(28) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639.
Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980,
72, 650.
(10) Ball, D. W.; Hauge, R. H.; Margrave, J. L. High Temp. Sci. 1988,
25, 95.
(11) Ball, D. W.; Hauge, R. H.; Margrave, J. L. Inorg. Chem. 1989, 28,
1599.
(12) Howard, J. A.; Joly, H. A.; Edwards, P. P.; Singer, R. J.; Logan,
D. E. J. Am. Chem. Soc. 1992, 114, 474.
(13) Lanzisera, D. V.; Andrews, L. J. Phys. Chem. A 1997, 101, 5082.
(14) Himmel, H. J.; Downs, A. J.; Greene, T. M. J. Am. Chem. Soc.
2000, 122, 9793.
(15) Zhang, L. N.; Dong, J.; Zhou, M. F. J. Phys. Chem. A 2000, 104,
8882. Zhou, M. F.; Zhang, L. N.; Shao, L. M.; Wang, W. N.; Fan, K. N.;
Qin, Q. Z. J. Phys. Chem. A 2001, 105, 5801.
(16) Zhou, M. F.; Zhang, L. N.; Dong, J.; Qin, Q. Z. J. Am. Chem. Soc.
2000, 122, 10680. Zhou, M. F.; Dong, J.; Zhang, L. N.; Qin, Q. Z. J. Am.
Chem. Soc. 2001, 123, 135.
(29) Wachter, J. H. J. Chem. Phys. 1970, 52, 1033. Hay, P. J. J. Chem.
Phys. 1977, 66, 4377.
(30) Head-Gordon, M.; Pople, J. A.; Frisch, M. Chem. Phys. Lett. 1988,
153, 503.
(31) Suzer, S.; Andrews, L. J. Chem. Phys. 1987, 87, 5131.
(32) Nelander, B.; Nord, L. J. Phys. Chem. 1982, 86, 4375.
(33) Zhou, M. F.; Zhang, L. N.; Lu, H.; Shao, L. M.; Chen, M. H. J.
Mol. Struct., in press.
(34) Khabashesku, V. N.; Kudin, K. N.; Margrave, J. L.; Fredin, L. J.
Organmet. Chem. 2000, 595, 248.
(35) Ismail, Z. K.; Hauge, R. H.; Fredin, L.; Kauffman, J. W.; Margrave,
J. L. J. Chem. Phys. 1982, 77, 1617.
(36) Elhanine, M.; Farrenq, R.; Guelachvili, G. J. Chem. Phys. 1991,
94, 2529.
(37) Withnall, R.; Andrews, L. J. Phys. Chem. 1985, 89, 3261.
(38) Zhou, M. F.; Chen, M. H.; Lu, H., to be published.
(17) Shao, L. M.; Zhang, L. N.; Lu, H., Chen, M. H.; Zhou, M. F. Chem.
Phys. Lett. 2001, 343, 178.