. 2023 Oct 13;14(42):9472–9478. doi: 10.1021/acs.jpclett.3c02280

Table 2. Conformational Stability and Population of Morpholine Were Determined via Two-Photon IR + VUV PI and IR Resonant VUV-PI Measurements.

method	T (K)	K (har)^a	K (anh)^a	ΔH (har)^b ( cm^–1)	ΔH (anh)^b ( cm^–1)
IR resonant VUV-PI	166	0.451	0.207	103 ± 12	192 ± 12
IR resonant VUV-PI	128	0.372	0.171	97 ± 10	166 ± 10
cal^c	0			255
UV-PD^d	mol beam			180 ± 50
IR^e	350–490			175 ± 35
two-photon IR + VUV-PIE^f	128	0.323		109 ± 4

Peak area ratios (I_Ax/I_Eq) between two conformers divided by the IR absorption probabilities calculated for 22 modes of the Chair-Ax and Chair-Eq conformers with harmonic and anharmonic potentials using B3LYP/aug-cc-pVTZ.

Enthalpy difference (E_Ax – E_Eq) using the simple equation ΔH = −RT ln K + TΔS, where K was estimated by the relative populations and ΔS was determined by quantum chemical calculations.

Difference of the calculated adiabatic ionization energy [ΔAIE_cal(Eq–Ax)].

Enthalpy difference (E_Ax – E_Eq) from ref (45).

Enthalpy difference (E_Ax – E_Eq) from ref (9).

The difference between the measured adiabatic ionization energies [ΔAIE(Eq–Ax)] of the Chair-Eq and Chair-Ax conformers equaled the relative energy of the Chair-Ax conformer, resulting from the existence of only the Chair-Ax-like⁺ ^• conformer in the D₀ state.