A Theoretical and Mass Spectrometry Study of Dimethyl Methylphosphonate: New Isomers and Cation Decay Channels in an Intense Femtosecond Laser Field

Abstract

Using both mass spectrometry with intense femtosecond laser ionization and high-level computational methods, we have explored the structure and fragmentation patterns of dimethyl methylphosphonate (DMMP) cation. Extensive search of the geometries of both neutral and positively charged DMMP yields new isomers that are appreciably lower in total energy than those commonly synthesized using the Michaelis-Arbuzov reaction. The stability of the standard isomer with CHPO(OCH) topology is found to be due to the presence of high barriers to isomer interconversion that involves several transition states. Our femtosecond laser ionization experiments show that the relative yields of the major dissociation products as a function of peak laser intensity correlate well with the theoretical estimates for the energies of the DMMP decay via various channels. In contrast, the peak laser intensities required for observation of minor dissociation products exhibit no correlation with the computed decay energies, which suggests that barrier heights and/or excited electronic states of DMMP determine its preferred fragmentation pathways in an intense femtosecond laser field.

Probing Coherent Vibrations of Organic Phosphonate Radical Cations with Femtosecond Time-Resolved Mass Spectrometry

“…To predict whether similar geometrical changes occur in DEMP and DIMP, the neutral and cation geometries were computed at the B3LYP/6-311+G* level. The computed geometries of DMMP agree with previous results [18,31,33] and serve to benchmark the method for DEMP and DIMP. Figure 6 depicts the neutral and cation geometries of DMMP, DEMP, and DIMP with the P=O, P–O, P–C, and C−O bond lengths and O–P–O angle labeled.…”

“…While the m / z = 109 and 93 structures result from direct cleavage of a methyl and methoxy group, respectively, the m / z = 94 and 79 structures involve hydrogen atom migration to the phosphate oxygen atom. This hydrogen migration has been likened to keto-enol isomerization [25] (Scheme 3) and has a transition state barrier of 0.42 eV from the relaxed ion geometry, with only an additional 0.03 eV required to lose CH 2 O and form PO 2 C 2 H 7 + according to our recent calculations [31]. The present FTRMS results indicate that this dissociation pathway can be induced by probe excitation of the parent cation.…”

“…The dissociation products analyzed in Figure 3 have been observed in numerous mass spectrometry studies of DMMP [23,24,25,26,27,28,29,30,31]. Their assigned structures based on these studies and our recently reported theoretical calculations [31] are given in Scheme 2. While the m / z = 109 and 93 structures result from direct cleavage of a methyl and methoxy group, respectively, the m / z = 94 and 79 structures involve hydrogen atom migration to the phosphate oxygen atom.…”

Conserved Vibrational Coherence in the Ultrafast Rearrangement of 2-Nitrotoluene Radical Cation

“…The expanded beam has maximum energy ranging from 50 − 150 µJ depending on the wavelength, and can reach focal peak intensities exceeding 4 × 10 14 W cm −2 . 43 The probe beam is directed to a retroreflector placed on a motorized translation stage (ThorLabs, Inc), attenuated with a variable neutral density filter, and frequency-doubled with a β-barium borate (BBO) crystal. The maximum probe energy is 10 µJ at 650 nm.…”

“…Pump and probe beams are recombined on a dichroic mirror and focused with a f = 20 cm fused silica biconvex lens into an ultrahigh vacuum chamber (base pressure 2 × 10 −9 torr) coupled to a custom-built linear time-of-flight mass spectrometer (TOFMS) described previously. 42,43 TOF-MS chamber Pump-probe measurements on 2-NT were taken with a pump wavelength of 1300 nm at intensities of 1 × 10 14 W cm −2 and 2 × 10 14 W cm −2 , as determined by the yield of Xe n+ ions using the method of Hankin. 45 The 650 nm probe intensity was determined through measurement of the energy using a pyroelectric power meter (PM-USB, Coherent, Inc.) and the beam waist using a CMOS camera (DCC1645M, ThorLabs, Inc.).…”