Gas-Phase Characterization of Adipic Acid, 6-Hydroxycaproic Acid, and Their Thermal Decomposition Products by Rotational Spectroscopy

We report the spectroscopic investigation of two bifunctional aliphatic carboxylic acids, namely, adipic acid and 6-hydroxycaproic acid, in the gas phase by combining high-resolution rotational spectroscopy and supersonic expansions. Their pure rotational spectra were successfully identified and characterized. However, due to the low thermal stability of these two chemicals, the measured rotational spectra were significantly congested with transitions corresponding to their decomposition products upon heating. We observed cyclopentanone and adipic anhydride in the spectrum of adipic acid and ε-caprolactone and its monohydrate in the spectrum of 6-hydroxycaproic acid. On the basis of the distinct fingerprints of both carboxylic acids and a series of their decomposition products, the spectra were analyzed in a time-segmented manner. This provides valuable insights into the thermal decomposition mechanisms of these two samples over time, which highlights the robustness of microwave spectroscopy as a potent tool for analyzing complex chemical mixtures in a species-, isomer-, and conformer-selective way.


Isomers
∆E ZP E /kJ mol   .Each spectrum was collected with 2 × 10 5 FID acquisitions.The rotational state are denoted as JK a K c , where J represents the total angular momentum, K a and K c are the projections of the angular momentum onto the principal molecular axes a and c.Note that measurements at 160 and 180 °C were also performed but not successful because the sample underwent rapid reactions, leading to the blockage of the nozzle orifice by decomposition products after a short period.

1. 1 Figure S1 :
Figure S1: Molecular geometries of the energetically low-lying conformers of adipic acid (AA) within an energy window of 5 kJ/mol, calculated at the B3LYP-D4/def2-QZVP level of theory.The relative energies are corrected with zero-point energies and that of AA-I is set to 0 kJ/mol.The AA-I structure shows the atom labelling.The corresponding Cartesian coordinates are available in Tables S11-S19 in Section 3.

Figure S2 :
Figure S2: Molecular geometries of the two most energetically stable conformers of adipic anhydride (AAD), calculated at the B3LYP-D4/def2-QZVP level of theory.The relative energies are corrected with zero point energies.In the geometry of AAD-I, the C 2 axis of molecular symmetry is displayed.The corresponding Cartesian coordinates are available in Tables S20-S21 in Section 3.

Figure S3 :Figure S4 :
Figure S3: Decarboxylation reaction pathways of AAD-I (a) and AAD-II (b) to the products of CO 2 + cyclopentanone (CPT), calculated through the nudged elastic band (NEB) method 1 at the B3LYP-D4/def2-QZVP level of theory using the ORCA 4.2.1 program package.

Figure S5 :
Figure S5: Molecular geometries of the energetically low-lying conformers of the monohydrated ε-caprolactone (ε-CL-1w) within an energy window of 5 kJ/mol, calculated at the B3LYP-D4/def2-QZVP level of theory.The relative energies are corrected with zero-point energies and that of ε-CL-1w-I is set to 0 kJ/mol.The corresponding Cartesian coordinates are available in Tables S24-S25 in Section 3.

Figure S6 :
Figure S6: Rotational transitions of ε-caprolactone (a) and 6-hydroxycaproic acid (b), monitored at different sample temperatures (°C).Each spectrum was collected with 2 × 10 5 FID acquisitions.The rotational state are denoted as JK a K c , where J represents the total angular momentum, K a and K c are the projections of the angular momentum onto the principal molecular axes a and c.Note that measurements at 160 and 180 °C were also performed but not successful because the sample underwent rapid reactions, leading to the blockage of the nozzle orifice by decomposition products after a short period.

Table S3 :
Experimental spectroscopic constants for AA-III, fitted with Watson's S -reduction Hamiltonian in its I r representation using the Pickett's SPFIT program.

Table S4 :
Experimental spectroscopic constants for AAD-I and AAD-II, fitted with Watson's A-reduction Hamiltonian in its I r representation using the Pickett's SPFIT program.

Table S11 :
Cartesian coordinates for the equilibrium structure of conformer I of adipic acid (AA-I) optimized at the B3LYP-D4/def2-QZVP level of theory.

Table S12 :
Cartesian coordinates for the equilibrium structure of conformer II of adipic acid (AA-II) optimized at the B3LYP-D4/def2-QZVP level of theory.

Table S13 :
Cartesian coordinates for the equilibrium structure of conformer III of adipic acid (AA-III) optimized at the B3LYP-D4/def2-QZVP level of theory.

Table S14 :
Cartesian coordinates for the equilibrium structure of conformer IV of adipic acid (AA-IV) optimized at the B3LYP-D4/def2-QZVP level of theory.

Table S15 :
Cartesian coordinates for the equilibrium structure of conformer V of adipic acid (AA-V) optimized at the B3LYP-D4/def2-QZVP level of theory.

Table S16 :
Cartesian coordinates for the equilibrium structure of conformer VI of adipic acid (AA-VI) optimized at the B3LYP-D4/def2-QZVP level of theory.

Table S17 :
Cartesian coordinates for the equilibrium structure of conformer VII of adipic acid (AA-VII) optimized at the B3LYP-D4/def2-QZVP level of theory.

Table S18 :
Cartesian coordinates for the equilibrium structure of conformer VIII of adipic acid (AA-VIII) optimized at the B3LYP-D4/def2-QZVP level of theory.

Table S19 :
Cartesian coordinates for the equilibrium structure of conformer IX of adipic acid (AA-IX) optimized at the B3LYP-D4/def2-QZVP level of theory.

Table S20 :
Cartesian coordinates for the equilibrium structure of conformer I of adipic anhydride (AAD-I) optimized at the B3LYP-D4/def2-QZVP level of theory.

Table S22 :
Cartesian coordinates for the equilibrium structure of conformer I of ε-

Table S26 :
Cartesian coordinates for the equilibrium structure of conformer I of 6hydroxycaproic acid (6-HCA-I) optimized at the B3LYP-D4/def2-QZVP level of theory.
4.1 Frequency list of adipic acid.