Metastable States of Dimethylammonium, (CH3)2NH2•

Hypervalent dimethylammonium radical, (CH 3 ) 2 NH 2 •, and its deuterium-labeled isotopomers (CH 3 ) 2 ND 2 •, (CH 3 ) 2 NHD •, (CD 3 ) 2 NH 2 •, and (CD 3 ) 2 ND 2 • were generated as transient species by collisional neutralization of their cations in the gas phase and studied by neutralization−reionization mass spectrometry, laser photoexcitation, and ab initio theory. (CH 3 ) 2 ND 2 •, (CH 3 ) 2 NHD •, and (CD 3 ) 2 NH 2 • gave fractions of metastable species of ≥3.3 μs lifetimes, whereas (CH 3 ) 2 NH 2 • and (CD 3 ) 2 ND 2 • dissociated completely on the same time scale. Metastable (CD 3 ) 2 NH 2 • and (CH 3 ) 2 ND 2 • were photoexcited but not photoionized with the combined 488 and 514.5 nm lines from an Ar-ion laser. Ab initio calculations with effective PMP4(SDTQ)/6-311++G(3df,2p) identified the (X̃) 2 A 1 ground state of vertical ionization energy, IE v = 3.70 eV. RRKM calculations on the ab initio potential energy surface of the (X̃) 2 A 1 state predicted predominant N−H and N−D bond dissociations but did not allow for competitive loss of CH 3 or CD 3 . The four lowest excited states of (CH 3 ) 2 NH 2 •, (Ã) 2 B 1, (B̃) 2 A 1, (C̃) 2 B 2, and (D̃) 2 A 1, were characterized by CIS/6-311++G(3df,2p) calculations, and their vertical ionization energies were calculated as 2.86, 2.57, 2.48, and 1.82 eV, respectively. The excited states were calculated to be strongly bound with respect to N−H bond dissociations. The N−C bond dissociations were interpreted by potential energy surface crossing of the B̃ and à states and transitions via conical intersection to the dissociative ground state.