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

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 = 1868.252 cm-1 (a1 symmetric N-O stretch) 5 = 1789.1 cm-1 (b2 antisymmetric N-O stretch)

NO dimer Four low frequency intermolecular vibrations, observed only recently in the gas phase 2 = 239.363 cm-1 (a1 symmetric bend) 3 = 134.503 cm-1 (a1 intermolecular stretch) 4 = 117 cm-1 (?) (a2 out-of-plane torsion, infrared inactive) 6 = 429.140 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

some previous hi-res spectroscopy NO dimer some previous hi-res spectroscopy Pure rotation Western, Langridge-Smith, Howard, & Novick, Mol. Phys. 44 (1981) 145-160 Kukolich, J. Mol. Spectrosc. 98 (1983) 80-86; Kukolich & Sickafoose, Mol. Phys. 89 (1996) 1659-1661 Brookes, McKellar, & Amano, J. Mol. Spectrosc. 185 (1997) 153-157 Infrared absorption Matsumoto. Y. Ohshima, M. Takami, J. Chem. Phys. 92 (1990) 937-942 [1 band, diode laser + jet] Howard & McKellar, Mol. Phys. 78 (1993) 55-72 [1 band, FTIR + long path cooled gas cell] Watson & McKellar, Can. J. Phys. 75 (1997) 181-189 [5 band, FTIR + long path cooled gas cell] East, McKellar, & Watson, J. Chem. Phys. 109 (1998) 4378-4383 [far-IR, FTIR + long path cooled gas cell] McKellar & Watson, J. Mol. Spectrosc. 194 (1999) 229-235 [far-IR, FTIR + long path cooled gas cell] Infrared photodissociation Hetzler, Casassa, & King, J. Phys. Chem. 95 (1991) 8086-8095 Raman (gas phase) Fernández, Tejeda, Ramos, Howard, & Montero, J. Mol. Spectrosc. 194 (1999) 278-280

NO dimer Experimental details Bomem DA3 Fourier transform IR spectrometer resolution ~ 0.06 cm-1 1500 – 1800 cm-1: KBr beamsplitter + 4.2 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

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

5 + 2 combination band

1 + 2 combination band

25 overtone band

(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)

Is this the 5 + 4 combination band?

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 ....

State (v1v2v3v4v5v6) Energy (cm-1) 3 001000 134.503 4 000100 155.5 2 010000 239.363 23 002000 254.11 3 + 4 001100 289. 2 + 3 001010 351.377 33 003000 359.7 6 000001 429.140 5 000010 1789.098 1 100000 1868.252 5 + 3 1933.81 5 + 4 000110 1964. 5 + 2 010010 2035.43 1 + 2 110000 2107.68 5 + 23 002010 2063.15 5 + 2 + 3 011010 2159.4 25 000020 3557.1 1 + 5 100010 3626.54 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”

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 3557.1 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