Article pubs.acs.org/JPCA
Dimethylamine Addition to Formaldehyde Catalyzed by a Single Water Molecule: A Facile Route for Atmospheric Carbinolamine Formation and Potential Promoter of Aerosol Growth Matthew K. Louie,† Joseph S. Francisco,‡ Marco Verdicchio,§ Stephen J. Klippenstein,*,§ and Amitabha Sinha*,† †
Department of Chemistry and Biochemistry, University of California−San Diego, La Jolla, California 92093-0314, United States Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, United States § Argonne National Laboratory, Chemical Sciences and Engineering Division, Argonne, Illinois 60439-4837, United States ‡
S Supporting Information *
ABSTRACT: We use ab initio calculations to investigate the energetics and kinetics associated with carbinolamine formation resulting from the addition of dimethylamine to formaldehyde catalyzed by a single water molecule. Further, we compare the energetics for this reaction with that for the analogous reactions involving methylamine and ammonia separately. We find that the reaction barrier for the addition of these nitrogen-containing molecules onto formaldehyde decreases along the series ammonia, methylamine, and dimethylamine. Hence, starting with ammonia, the reaction barrier can be “tuned” by the substitution of an alkyl group in place of a hydrogen atom. The reaction involving dimethylamine has the lowest barrier with the transition state being 5.4 kcal/mol below the (CH3)2NH + H2CO + H2O separated reactants. This activation energy is significantly lower than that for the bare reaction occurring without water, H2CO + (CH3)2NH, which has a barrier of 20.1 kcal/mol. The negative barrier associated with the single-water molecule catalyzed reaction of dimethylamine with H2CO to form the carbinolamine (CH3)2NCH2OH suggests that this reaction should be energetically feasible under atmospheric conditions. This is confirmed by rate calculations which suggest that the reaction will be facile even in the gas phase. As amines and oxidized organics containing carbonyl functional groups are common components of secondary organic aerosols, the present finding has important implications for understanding how larger, less volatile organic compounds can be generated in the atmosphere by combining readily available smaller components as required for promoting aerosol growth.
I. INTRODUCTION Amines are important compounds that are of interest to a wide range of researchers exploring such diverse areas as the formation of amino acids in interstellar space,1−8 schemes for carbon capture,9−11 and aerosol formation in the earth’s troposphere.12−35In the earth’s atmosphere, amines arise from both anthropogenic and biogenic sources including animal husbandry, automobile exhaust, marine organisms, and biomass burning.13,14,33 The atmospheric removal of amines results from their reactions with either nitric or sulfuric acid to form the corresponding salts or through oxidation by OH, O3, and NO3.33 The concentrations of amines vary widely depending on location and proximity to the source with their ambient concentrations typically being at least an order of magnitude smaller than that of ammonia.13,15 Many studies have revealed the important role that amines play in the chemistry of the earth’s atmosphere, especially in promoting aerosol growth.12−35 Oxidation of amines via OH and NO3 radicals, acid−base reactions of amines with H2SO4 or HNO3, and formation of nitrosamines are a few examples of atmospheric amine chemistry that have thus far been studied.11,14,33 The © XXXX American Chemical Society
atmospheric chemistry of amines has been reviewed recently by Nielsen et al., who emphasized the need for additional kinetic and mechanistic data.11 One reaction that has not received extensive attention in the context of atmospheric chemistry is the reaction of amines with carbonyl compounds to form carbinolamines (also called hemiaminal). The rather general case involving a secondary straight chain amine and formaldehyde is illustrated by the following reaction: NHRR′ + H 2CO → NRR′CH 2OH
(1)
In the above equation, R and R′ can either be hydrogen atoms or alkyl groups, and cases involving carbonyl molecules more complex than formaldehyde are also possible. Although several laboratory measurements investigating the role of amines in stimulating aerosol growth have reported the Special Issue: James G. Anderson Festschrift Received: May 21, 2015 Revised: September 25, 2015
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DOI: 10.1021/acs.jpca.5b04887 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
square (rms) forces were
