385
J. Phys. Chem. 1993.97, 385-390
Unimolecular Decay of Energy-Selected Dimetbylformamide Cations: A Combined Molecular Orbital and RRKM Analysis John Riley and Tomas h e r ' Chemistry Department, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290 Received: July 28, 1992; In Final Form: October 20, 1992
The unimolecular dissociation chemistry of N,N-dimethylformamide cations, (CH&NCHO+, has been investigated by photoelectron-photoion coincidence in the energy range 11.1 1-1 1.49 eV. The measured rates for CHO and CH3 loss were modeled with RRKM statistical theory. The vibrational frequencies for both the molecular ion and two transition states were obtained from ab initio MO calculations so that the only adjustable parameters in the RRKM analysis were the activation energies. This analysis shows that these apparently simple dissociation reactions proceed via substantial reverse activation energies and tight transition states. Ab initio MO calculations (3-21G' basis set) of the transition states suggest that these reactions proceed via an intramolecular (not associated with the expelled radical) hydrogen atom transfer to produce the CH3NHCH2+ and CHjNHCO+ product ions, rather than (CH3)2N+ and CH3NCHO+ expected from simple loss of CHO and CH3. The measured kinetic energy release was found to be only slightly greater than that predicted by the statistical theory.
Iatroduction The developmentof mass spectrometry techniques that enable the study of large peptideshas been accompaniedby recent interest in the study of small model amide compounds.'4 Of these, the simplest fully substituted amide, dimethylformamide (DMF),
has generated questions concerning the mechanisms involved in fragm~ntation.~ DMF produces fragment ions consistent with simplebond cleavages,for exampleloss of CHO and CH3neutrals. However, in the collision-induced dissociation (CID) spectrum of (CH3)2NCDO+,the CHO loss intensity was greater than that for CDO loss in contrast to exclusive loss of CDO in the metastable spectra. This led Ann and Adams4 to conclude that slow direct cleavage of the N-CDO bond was effectively competing with rapid hydrogen rearrangement. Alsoof interest are the structures of the ionic speciesproduced in these reactions. For example, the electron impact appearance energy for C&N+ + CHO5 is larger than the product energy for several C&N+ isomers. In 1977, Loudon and Webb6 suggested that CHO loss from DMF cations was not a simple bond cleavage. This was based on an appearance energy of 11.60 eV that is 1.30 eV lower than that predicted using bond dissociation energy ideas. These workers argued that the a hydrogen rearrangement followed considerable N-CHO stretching, leading effectively to a product ion isomerization:
This was consistent with an earlier study? in which it was shown that (CH3)2N+and CH,NHCH2+ formed from various alkylamines produce a common ion structure. CID experiments with isotopically labeled precursors eliminated (CH3)2N+ as this common structure,and CH3NHCH2+was consideredmore stable on the basis of bonding characteristics. The heats of formation of C&N+ ions, determined by electronimpact appearanceenergy meas~rements~.~ amfirmed that CH,NHCH2+ is the lowest energyisomer. Thesestuditsalsoshowed that reactionsexpected to yield (CH1)2N+actually produced the CH3NHCH2+ion. Ab 0022-3654/58/2097-0385~04.00/0
initio results, although differing in absolute energy, agreed with the ordering of CzHaN+ isomer energies.10 Simple bond cleavage reactions are generally associated with the breaking of single bonds. However, in neutral DMF, CHO rotation about the N - C bond has a barrier on the order of 1 eV, and the N-CHO bond distance is intermediate between those of C - 0 and N-CH3,11a trend confirmed by ab initio calculated bond lengths for the DMF cations3 Therefore, the simple N-CHO bond break would require cleavage of a partial double bond. In order to address the dissociation mechanisms and product ion structures for low-energy DMF fragmentations, metastable dissociation rates and kinetic energy releases (KERs) for DMF cations have been measured by photoelectron-photoion coincidence (PEPICO), a technique that couples energy-selective photoionization with time of flight (TOF) mass spectrometry. The two lowest energy reactions, loss of CHO and CH1, were studied in the energy range 11.1 1-1 1.49 eV.
Experimental Seetion The PEPICO experiment has been described extensively12 but will be briefly outlined here. Vacuum ultraviolet radiation from an Hzdischarge ionizes the sample in the uniform field (20 V/cm) source of a TOF mass spectrometer. Photoelectrons are accelerated into a steradiancy analyzer13where geometrical energy discrimination allows selected transmission of electrons with initially zero kinetic energy (total resolution, Ahu A&, about 20 meV). These zero energy electrons provide a start pulse to a time to pulse height converter, and detection of a coincident ion signals the stop. The TOF distributions were collected on a multichannel pulse height analyzer. DMFvapor was introduced by expansion from a 300-pm nozzle, and the resulting jet passed %rough a 1-mm skimmer into the photoionization region. The room-temperature vapor pressure of DMF was insufficient to produce an internally cold neutral beam, so argon was bubbled through the liquid, giving a total backing pressure of 15 Torr. TOF spectra recorded at 120 Torr showed no difference in the dissociationrates that would indicate insufficient cooling at the 15 Torr backing p m u r e . In addition, the partial pressure of DMF in the expansion was reduced while the total backing pressure was kept constant to determine the role of DMF cluster formation. A 1-mm-i.d. effusiveneedle was used to record thermal spectra to subtract out a 15% thermal background in the molecular beam data.
+
Q 1993 American Chemical Society
386 The Journal of Physical Chemistry, Vol. 97, No. 2, 1993
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