An international team of scientists from France and Spain has accurately determined the gas-phase structure of alpha-pinene, a molecule emitted into the atmosphere mainly by pine forests, and also, in lower quantities, by Amazonian and tropical forests. The results appear in the Journal of Chemical Physics.
Gas-phase structure of alpha-pinene was experimentally unveiled, using Fourier transform microwave spectroscopy and quantum chemical calculations. Image credit: Elias M. Neeman.
Among biogenic volatile organic molecules is a class of compounds called monoterpenes, which react with ozone, hydroxyl radicals, nitrogen oxides, and other gases in the atmosphere.
These reactions create pollutants and aerosols, which, for example, can seed the condensation of more clouds and help cool the climate.
The main monoterpene is alpha-pinene, of which forests release about 50 trillion grams each year into the atmosphere’s lowest layer, the troposphere.
“What you smell in the forest are biogenic volatile organic molecules emitted into the atmosphere,” said Dr. Thérèse Huet, a physicist at the University of Lille in France.
“Determining the structure of these molecules is paving the way for future spectroscopic detection and atmospheric chemistry modeling.”
To accurately predict how alpha-pinene reacts in the troposphere, and thus how it affects climate and air quality, scientists need a detailed understanding of its molecular structure.
Alpha-pinene in its solid phase has an already determined bicylic or two-ring structure. In the troposphere, however, alpha-pinene is a gas and is found to have notable changes in structure due to forces exerted by the crystal structure.
Because alpha-pinene exists in the gas phase only at low concentrations, requiring highly sensitive experimental techniques, no one had previously identified its gas-phase structure.
“Up to now, structure determination of complex molecules such as monoterpenes was only possible in the condensed phase,” Dr. Huet said.
Dr. Huet and co-authors developed methods sensitive enough to determine the structure of monoterpene gases.
They identified quantum parameters called rotational constants that best describe the data with no structural assumptions.
They repeated this analysis for all naturally occurring isotopic versions of alpha-pinene, in which carbon-13 isotopes replace different carbon-12 isotopes in the molecule.
This set of rotational constants describes the complete structure of alpha-pinene gas.
“Our technique can also be used for other monoterpenes,” Dr. Huet noted.
“Now that we have the structure of alpha-pinene, we are better able to model the first steps of nature’s aerosol formation in the atmosphere.”
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Elias M. Neeman et al. 2017. The gas phase structure of α-pinene, a main biogenic volatile organic compound featured. Journal of Chemical Physics 147 (21): 214305; doi: 10.1063/1.5003726
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