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Compare vibrational frequencies in CCCBDB for CH3NO2 (Methane, nitro-)

HF/CEP-31G*

18 11 26 13 55
Frequency in cm-1
  Calculated   Experimental
Mode Number Symmetry Frequency Scaled
Frequency
Harm.
diff
Fund.
diff
Symmetry Harmonic
Frequency
Fundamental
Frequency
Comment
1 A' 3392 3046   1 A'   3045  
2 A' 3289 2954   -20 A'   2974  
3 A' 1673 1502   105 A'   1397  
4 A' 1607 1443   9 A'   1434  
5 A' 1554 1396   16 A'   1380  
6 A' 1259 1130   -1 A'   1131  
7 A' 1031 926   8 A'   918  
8 A' 723 650   -7 A'   657  
9 A' 673 604   1 A'   603  
10 A" 3423 3074   -6 A"   3080  
11 A" 1847 1658   75 A"   1583  
12 A" 1596 1433   23 A"   1410  
13 A" 1223 1099   3 A"   1096  
14 A" 514 462   -13 A"   475  
15 A" 17i 15i    A"     internal rotation, almost free rotor
The calculated vibrational frequencies were scaled by 0.898
Nitromethane is essentially a free rotor. The calculations produce one of three conformations: 1) the NO2 plane perpendicular to a C-H bond, Cs symmetry; 2) The NO2 plane lined up with a C-H bond, Cs symmetry; 3) The NO2 plane at some small angle away from a C-H bond (C1 symmetry). Note that conformation 1 has 9 A' modes and 6 A" modes, conformation 2 has 10 A' modes and 5 A" modes, and conformation 3 has 15 A modes.

See section Calculated; Vibrations; Scale Factors; Set scaling factors to change the scale factors used here.
See section Calculated; Vibrations; Scale Factors; Calculate a scale factor to determine the least squares best scaling factor.
See section Calculated; Vibrations; Scale Factors; Scale factor uncertainty for information on where the above scaling factor is from.