1. K.-X. AuYong, J.M. King, A.R.W. McKellar, & J.K.G. Watson
Infrared combination and difference bands of the NO dimer
K.-X. AuYong, J.M. King, A.R.W. McKellar, & J.K.G. Watson
Steacie Institute for Molecular Sciences National Research Council of Canada

2. NO dimer Planar C2v structure; Singlet ground state
R (N-N) = 2.27 Å
R (N-O) = 1.16 Å
θ (N-N-O) = 97º O O
Planar C2v structure; Singlet ground state
Binding energy ~ 700 cm-1
(strong for van der Waals, but very weak relative to ‘normal’ bond)
Two high frequency mid-IR vibrations
1 = cm-1 (a1 symmetric N-O stretch)
5 = cm (b2 antisymmetric N-O stretch)

3. NO dimer
Four low frequency intermolecular vibrations, observed only recently in the gas phase
2 = cm-1 (a1 symmetric bend)
3 = cm-1 (a1 intermolecular stretch)
4 = 117 cm-1 (?) (a2 out-of-plane torsion, infrared inactive)
6 = cm-1 (b2 antisymmetric bend)
In the present work, we observe and assign a number of combination, difference, and overtone bands in the mid-infrared region (1500 – 3800 cm-1), and also obtain a new value for 4

4. some previous hi-res spectroscopy
NO dimer
some previous hi-res spectroscopy
Pure rotation
Western, Langridge-Smith, Howard, & Novick, Mol. Phys. 44 (1981)
Kukolich, J. Mol. Spectrosc. 98 (1983) 80-86; Kukolich & Sickafoose, Mol. Phys. 89 (1996)
Brookes, McKellar, & Amano, J. Mol. Spectrosc. 185 (1997)
Infrared absorption
Matsumoto. Y. Ohshima, M. Takami, J. Chem. Phys. 92 (1990) [1 band, diode laser + jet]
Howard & McKellar, Mol. Phys. 78 (1993) [1 band, FTIR + long path cooled gas cell]
Watson & McKellar, Can. J. Phys. 75 (1997) [5 band, FTIR + long path cooled gas cell]
East, McKellar, & Watson, J. Chem. Phys. 109 (1998) [far-IR, FTIR + long path cooled gas cell]
McKellar & Watson, J. Mol. Spectrosc. 194 (1999) [far-IR, FTIR + long path cooled gas cell]
Infrared photodissociation
Hetzler, Casassa, & King, J. Phys. Chem. 95 (1991)
Raman (gas phase)
Fernández, Tejeda, Ramos, Howard, & Montero, J. Mol. Spectrosc. 194 (1999)

5. NO dimer Experimental details
Bomem DA3 Fourier transform IR spectrometer
resolution ~ 0.06 cm-1
1500 – 1800 cm-1: KBr beamsplitter K Ge:Cu detector
2000 – 3800 cm-1: CaF2 beamsplitter + 77 K InSb detector
5 meter multi-pass cooled gas cell
32 (or 36) traversals for a total path of 160 (or 180) meters
NO sample pressure 16 to 22 Torr, cell cooled to 99 K

7. calculated (NO monomer)
5 + 3 combination band
observed spectrum
calculated (NO monomer)
calculated
calculated
calculated

8. 5 + 2 combination band

9. 1 + 2 combination band

10. 25 overtone band

11. (much stronger than 25, looks very similar to 5 fundamental)
1 + 5 overtone band
(much stronger than 25, looks very similar to 5 fundamental)

12. Is this the 5 + 4 combination band?

13. Reassignment of 4
Previously (1998) we assigned the feature at 1672 cm-1 as 5 - 4, which gives 4 = 1789 – 1672 = 117 cm-1
Now, we assign the feature at 1634 cm-1 as 5 - 4, which gives 4 = 1789 – 1634 = 155 cm-1
And, we assign the feature at 1964 cm-1 as 5 + 4. There is no other reasonable assignment for the 1964 cm-1 mystery feature!
The increase in 4 upon excitation of 5 (from 155 to 175) is not unreasonable ....

14. State
(v1v2v3v4v5v6)
Energy (cm-1) 3
001000 4
000100
155.5 2
010000
23
002000
254.11
3 + 4
001100
289.
2 + 3
001010
33
003000
359.7 6
000001 5
000010 1
100000
5 + 3
5 + 4
000110
1964.
5 + 2
010010
1 + 2
110000
5 + 23
002010
5 + 2 + 3
011010
2159.4
25
000020
3557.1
1 + 5
100010
21
200000
3723.7
The number of known vibrational states for (NO)2 is almost doubled by this work
Blue means “very certain”
Green means “less certain”

15. Conclusions
No evidence for “other” forms of NO dimer (e.g. trans structure)
The strongest observed combinations involve 5 and 3
The intermolecular frequencies 2, 3, and 4 increase upon excitation of 5. Understandable in terms of stronger bonding, shown by red shift (-87 cm-1) of 5 relative to NO monomer
We believe that 4 = 155 cm-1 (but difficult to be absolutely certain)
Hetzler, Casassa, & King’s (1991) value for 25 is 2.2 cm-1 too high (a laser calibration error?) The correct value is cm-1
The lifetime of the 5 state is increased by simultaneous excitation of 3 (may illustrate Le Roy’s rule: “all else being equal,” predissociation rates are proportional to frequency shifts squared)
We had fun using these simulation/fitting programs: David Plusquellic’s JB95 and Colin Western’s PGopher