16
Y. Zhao et al. / Chemical Physics 351 (2008) 13–18
940.6, 897.3 and 509.7 cmꢀ1 were produced, which exhibited
Xe
Ar
Ar
Ar
77.8
about the same isotopic spectral features as the 1109.3, 945.9/
943.5, 903.6/900.6 and 511.3 cmꢀ1 absorptions. These experimen-
tal observations led us to propose the reassignment of the 1109.3,
945.9, 903.6 and 511.3 cmꢀ1 absorptions to NbO4(Ar), a (g2-
O2)NbO2(Ar) complex, and the 1109.3, 943.5, 900.6 and
511.3 cmꢀ1 absorptions to NbO6, a (g2-O2)NbO2–OO complex. The
1110.7, 940.6, 897.3 and 509.7 cmꢀ1 absorptions appeared in xe-
non doping experiment are attributed to the NbO4(Xe) complex.
The O–O stretching and Nb–O2 stretching modes for NbO4(Ar)
and NbO6 can not be resolved experimentally.
83.9
2.936
3.156
Nb
2.925
1.747
O
Nb
Nb
106.3
1.763
O
O
1.767
O
115.1
113.6
O
O
NbO2, C2v
,
2A1
NbO2(Ar)2, C2v
,
2A1
NbO2(Ar)(Xe), Cs,
2A'
1.315
1.315
O
O
O
Xe
O
2.194
Xe
2.156
91.6
Nb
2.129
3.161
Theoretical calculations support the above assignments. The
NbO4 molecule was predicted to have a 2A2 ground state with
(g2–O2)NbO2C2v structure, in agreement with the previous report
[11]. On the basis of the calculated O–O bond length and the ob-
served O–O stretching frequency [24], NbO4 can be regarded as
[(Oꢀ2 )(NbO2)+], a side-on bonded superoxide complex. Present
DFT calculations also indicate that the NbO4 molecule is able to
coordinate another dioxygen or one noble gas atom in forming
the NbO4(X) (X = O2, Ar, Xe) complexes, which were predicted to
have a Cs(2A00) structure with the two oxygen atoms in superoxide
subunit being slightly inequivalent and is in the same plane with
the X unit, which is perpendicular to the ONbO plane (Fig. 5).
The NbO4 subunit is slightly distorted upon X coordination. The
Nb–X bond distances were predicted to be 2.385, 2.788 and
3.089 Å for NbO6, NbO4(Ar) and NbO4(Xe), respectively. As listed
in Table 3, the O–O stretching, symmetric and antisymmetric and
Nb–O2 stretching modes of NbO4 are slightly shifted upon Ar coor-
dination. When the Ar atom in NbO4(Ar) is replaced by O2, the
above-mentioned modes are red-shifted by 1.2, 3.1, 7.6 and
1.5 cmꢀ1, respectively, which are slightly larger than the experi-
mental values (2.4 and 3.0 cmꢀ1 for the two ONbO stretching
modes, the shifts of other two modes cannot be resolved). When
the Ar atom is replaced by Xe, the O–O stretching mode is pre-
dicted to be blue-shifted by 0.7 cmꢀ1, whereas the other three
modes are red-shifted by 7.0, 7.3 and 2.3 cmꢀ1, respectively. Exper-
2.385
114.5
Nb
Nb
O
1.771
O
1.201
O
1.743
O
116.6
1.749
O
110.9
110.7
O
O
O
2A1
NbO4, C2v,
2A2
NbO2(Xe)2, C2v
,
NbO6, Cs
,
4A'
1.316
O
1.316
O
O
O
2.197
2.193
2.128
2.129
Nb
Nb
Ar
2.788
1.747
O
1.750
O
Xe
3.089
110.9
110.8
O
O
NbO4(Ar), Cs,
2A"
NbO4(Xe), Cs,
2A"
Fig. 5. Optimized geometric parameters (bond lengths in angstrom and bond an-
gles in degree) of NbO2(Ar)n(Ng)2ꢀn (n = 0, 1, 2) and NbO4(X) (X = Ar, Xe and O2) at
the B3LYP levels of theory.
garded as ‘‘isolated” species. Consistent with the experimental
observations, quantum chemical calculations on the TaO2(Ar)2
complex converged to separated TaO2 and Ar atoms (with the
Ta–Ar distances larger than 4 Å), which indicates that TaO2 cannot
form stable noble gas complexes.
NbO4(X) (X = Ar, Xe, O2). The 1109.3, 945.9/943.5, 903.6/900.6
and 511.3 cmꢀ1 absorptions were previously assigned to the O–O
stretching, symmetric and antisymmetric ONbO stretching and
Nb–O2 stretching modes of NbO4 with the (g2-O2)NbO2 C2v struc-
ture at two matrix sites [11]. As can be seen in Fig. 1, the 943.5
and 900.6 cmꢀ1 absorptions increased readily on higher tempera-
ture annealing at the expense of the 945.9 and 903.6 cmꢀ1 absorp-
tions; When xenon is doped into argon, the 943.5 and 900.6 cmꢀ1
absorptions were absent but a group of new absorptions at 1110.7,
imentally, the O–O stretching mode is blue-shifted by 1.4 cmꢀ1
while the other three modes are red-shifted by 5.3, 6.3 and
1.5 cmꢀ1
,
.
TaO4(X) (X = Ar, Xe, O2). Similar absorptions at 1095.7, 950.5,
894.5 and 524.2 cmꢀ1 in the Ta + O2/Ar system, which were previ-
ously [11] attributed to TaO4 should be reassigned to the TaO4(Ar)
complex following the example of NbO4(Ar). The 946.3 889.4 and
522.4 cmꢀ1 absorptions increased readily on higher temperature
Table 3
Calculated vibrational frequencies (cmꢀ1) and intensities (km/mol) for the product molecules at the B3LYP level of theory
Molecule
Frequency (intensity, mode)
NbO2 (2A1)
954.1 (134, a1), 924.7 (264, b2), 296.1 (a1, 2)
NbO2(Ar)2 (2A1)
NbO2(Ar)(Xe) (2A0)
NbO2(Xe)2 (2A1)
NbO4 (2A2)
936.7 (60, a1), 898.1 (326, b2), 271.7 (10, a1), 106.7 (19, b1), 93.7 (1, a1), 48.0 (7, b2), 46.9 (0, a1), 39.5 (8, b1), 26.9 (0, a2)
930.8 (63, a0), 893.0 (324, a00), 2269.6 (12, a0), 105.0 (20, a0), 82.4 (1, a0), 51.7 (5, a00), 37.7 (1, a0), 36.0 (9, a0), 30.2 (1, a00)
925.4 (67, a1), 888.6 (321, b2) 269.4 (13, a1), 104.9 (22, b1), 72.7 (0, a1), 51.7 (6, b2), 43.3 (0, a2), 32.9 (11, b1), 26.0 (1, a1)
1207.0 (45, a1), 970.0 (117, a1), 952.0 (285, b2), 489.8 (5, b1), 480.3 (41, a1), 287.8 (0, a1), 197.8 (0, a2), 143.8 (7, b2), 77.9 (75, b1)
1210.8 (42, a0), 962.9 (106, a0), 941.5 (267, a00), 500.1 (29, a0), 450.3 (19, a0), 287.3 (2, a0), 188.3 (0, a00), 148.6 (5, a00), 141.1 (49, a0), 120.9 (29, a0),
79.9 (0, a00), 70.3 (0, a0)
NbO4(Ar) (2A00)
NbO4(Xe) (2A00)
NbO6 (4A0)
1211.5 (42, a0), 955.9 (103, a0), 934.2 (256, a00), 497.8 (33, a0), 444.6 (17, a0), 283.8 (3, a0), 183.7 (0, a00), 146.9 (4, a00), 140.7 (77, a0), 88.1 (1, a0),
72.0 (0, a00), 58.3 (0, a0)
1632.0 (4, a0), 1209.6 (50, a0), 959.8 (100, a0), 933.9 (262, a00), 498.6 (27, a0), 449.7 (19, a0), 287.5 (2, a0), 223.2 (4, a0), 191.3 (0, a00), 152.9
(42, 191.3)(0, a00), 151.5 (6, a00), 110.4 (0, a00), 107.2 (21, a0), 66.1 (8, 0)
TaO2 (2A1)
975.1 (62, a1), 927.9 (193, b2), 321.6 (6, a1)
TaO4 (2A00)
1182.6 (23, a0), 958.2 (71, a0), 916.2 (240, a00), 517.7 (12, a0) 435.5 (32, a0), 289.5 (3, a0), 200.4 (0, a00), 150.4 (5, a00), 98.9 (58, a0)
1189.2 (23, a0), 954.8 (70, a0), 912.2 (233, a00), 511.6 (16, a0) 428.4 (29, a0), 289.5 (4, a0), 200.3 (0, a00), 162.9 (44, a00), 148.9 (4, a0), 126.5 (18, a0),
80.2 (1, a00), 64.2 (0, a0)
TaO4(Ar) (2A00)
TaO4(Xe) (2A00)
TaO6 (4A0)
1192.1 (23, a0), 950.9 (70, a0), 907.9 (223, a00), 507.3 (18, a0) 423.3 (28, a0), 288.1 (4, a0), 197.4 (1, a00), 157.4 (60, a00), 147.5 (4, a0), 90.5 (1, a0),
71.8 (1, a00), 57.5 (1, a0)
1587.3 (50, a0), 1187.9 (49, a0), 944.3 (59, a0), 892.3 (205, a00), 505.4 (10, a0), 422.1 (24, a0), 287.3 (2, a0), 243.1 (0, a0), 199.5 (0, a00), 156.3 (34, a0),
146.9 (5, a00), 110.6 (1, a00), 109.6 (10, a0), 63.9 (1, a0), 10.4 (0, a00)