Probing the early stages of solvation of cis-pinate dianions by water, acetonitrile, and methanol: a photoelectron spectroscopy and theoretical study

Abstract

cis-Pinic acid is one of the most important oxidation products of α-pinene--a key monoterpene compound in biogenic emission processes. Molecular level understanding of its interaction with water in cluster formation is an important and necessary prerequisite for ascertaining its role in the aerosol formation processes. In this work, we studied the structures and energetics of the solvated clusters of cis-pinate (cis-PA(2-)), the doubly deprotonated dicarboxylate of cis-pinic acid, with H2O, CH3OH, and CH3CN by negative ion photoelectron spectroscopy and ab initio theoretical calculations. We found that cis-PA(2-) prefers being solvated alternately on the two -CO2(-) groups with increase of solvent coverage, a well-known solvation pattern that has been observed in microhydrated linear dicarboxylate dianion (DCn(2-)) clusters. Experiments and calculations further reveal an intriguing feature for the existence of the asymmetric type isomers for cis-PA(2-)(H2O)2 and cis-PA(2-)(CH3OH)2, in which both solvent molecules interact with only one of the -CO2(-) groups, a phenomenon that has not been observed in DCn(2-) water clusters and exhibits that the subtle effect of the rigid four-membered carbon ring brought on the cis-PA(2-) solvation. The dominant interactions between cis-PA(2-) and solvent molecules form bidentate O(-)...H-O H-bonds for H2O, O(-)...H-O and O(-)...H-C H-bonds for CH3OH, and tridentate O(-)...H-C H-bonds for CH3CN. The formation of inter-solvent H-bonds between H2O and CH3CN is found to be favorable in mixed solvent clusters, different from that between H2O and CH3OH. These findings have important implications for understanding the mechanism of cluster growth and the formation of atmospheric organic aerosols, as well as for rationalizing the nature of structure-function relationship of proteins containing carboxylate groups in various solvent environments.

Molecular Specificity and Proton Transfer Mechanisms in Aerosol Prenucleation Clusters Relevant to New Particle Formation

“…Figure 3 presents the NIPE spectra of cis-PA 2− (H 2 O) n (n = 0−4) clusters and their optimized low-lying structures. 26 A close inspection of the spectra unveils interesting odd−even variations with the water number for both EBEs and spectral band shapes. The spectra show that the odd number added H 2 O (first and third) shifts the spectrum to high EBE by ∼0.3 eV and broadens the spectral features by 30%, compared to a shift of ∼0.6 eV to high EBE by the even number added H 2 O (second and fourth).…”

“…Clusters with tailored compositions and precisely controlled sizes are ideal in providing microscopic information on macroscopic phenomena and offer the unique opportunity to untangle the complex intermolecular interactions and thermodynamics important to the formation of critical nuclei during NPF. − ,,− They are also tractable with first principle calculations, and in turn, the experimental data on those clusters can reinforce the improvement of theoretical models and methods.…”

Sulfuric acid and aromatic carboxylate clusters H2SO4·ArCOO−: Structures, properties, and their relevance to the initial aerosol nucleation

“…[8] We have been interested in characterizing the small pre-nucleation clusters relevant to NPF, especially organic acid containing clusters, by combining negative ion photoelectron spectroscopy (NIPES) and quantum chemical calculations. [30][31][32][33][34][35][36][37] For example, we have shown that succinic acid molecule, HO 2 C(CH 2 ) 2 CO 2 H, is capable of stabilizing the bisulfate ion HSO 4 − and its own conjugate base succinate ion, HO 2 C(CH 2 ) 2 CO 2 − , indicating significant thermodynamic advantage of organic molecules in promoting bisulfate clusters and organic aerosol particles formation. [30,34] Most recently, we have studied the deprotonated sulfuric acid-formic acid anionic cluster [H 2 SO 4 •HCOOH−H + ] − , and found that the charge delocalization accompanying formation of two strong hydrogen bonds is strong enough to compensate the proton affinity imbalance in forming H 2 SO 4 •HCOO − cluster.…”

Photoelectron spectroscopy of solvated dicarboxylate and alkali metal ion clusters, M+[O2C(CH2)2CO2]2−[H2O]n(M = Na, K;n= 1–6)

“…We tentatively assign it to higher energy quaternary [Na + , HDC 2 − , OH − , H 2 O] complex, hinging on the hypothesis that such complex may be kinetic trapped. [35][36][37][38][39]…”

“…However, it needs to be pointed out that all component ions in both quaternary and ternary complexes [Na + , HDC 2 − , OH − , H 2 O, DC 2 2− ] exist in our sprayed solution, and previous studies have shown that higher energy isomers can often exist even at low temperatures due to kinetic trapping. [35][36][37][38][39] Therefore the existence of such quaternary system remains a possibility, which may contribute to the weak low binding energy band.…”