The Line Width of the EPR Signal of Gaseous Nitric Oxide as Determined by Pressure and Temperature-Dependent X-band Continuous Wave Measurements

Abstract

The electron paramagnetic resonance (EPR) signal of gaseous nitric oxide (NO) has been measured by continuous wave X-band experiments at room temperature at gas pressures between 1 mbar and 60 mbar and at a gas pressure of 48 mbar at different low temperatures. A phenomenological spin Hamiltonian approach allows simulating each EPR signal of NO by changing only a single line width parameter. At room temperature, this line width depends linearly on the NO gas pressure which can be explained by kinetic gas theory. An effective collisional cross section has been determined by this way which is about twice as large as the known cross section for NO derived from viscosity measurements. Experiments with NO gas at low temperatures are consistent to the line width interpretation by kinetic theory. In total, the results demonstrate that in NO adsorption and desorption experiments at different temperatures and NO gas pressures below 60 mbar the amount of desorbed NO can simply be determined in situ by the line width of this signal, which one can obtain easily from a conventional simulation procedure.

EPR Insights into Switchable and Rigid Derivatives of the Metal–Organic Framework DUT-8(Ni) by NO Adsorption

“…18 These three lines have been collapsed into one line in the adsorption branch at T = 115 K as a result of the higher NO gas pressure. 20 This proves directly that more NO gas is desorbed at this temperature during the temperature driven adsorption than during the desorption. We have recently established a relation between the room temperature line width of the EPR signal of NO gas and its room temperature pressure.…”

“…We have recently established a relation between the room temperature line width of the EPR signal of NO gas and its room temperature pressure. 20 The homogenous Lorentzian peak-topeak line width of the room temperature EPR signal of NO gas in sample F62 has been determined by simulation to be ‫ܤߜ‬ NOgas = 11.6 ± 1.5 (S6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 16 One can estimate from this value, that the amount of adsorbed NO at T = 126 K is ܰ ads 126 K = 10.0 ± 1.0 µmol/mg (S3). This is clearly more than ܰ ads 127 K as determined for the equivalent flexible DUT-8(Ni) sample F860 (Table 1).…”

“…In these measurements the EPR signal of the lowest rotational level of the 2 Π 3/2 state of desorbed NO gas [18][19][20] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 12 maximum at T max = 132 ± 17 K. The decrease of I NOgas at higher temperatures expresses the depopulation of the corresponding rotational ground state of the 2 Π 3/2 state in combination with Curie's law. 22 At lower temperatures the signal intensity I NOgas decreases drastically until it approaches almost zero at about T des = 93 ± 11 K. The heating (desorption) branch of I NOgas has a maximum at T max = 149 ± 22 K. This value is larger than the value T max ≈ 132 K which has been approached during cooling.…”

“…X-band cw EPR measurements of these samples were performed at the temperature T = 123 K. The homogeneous Lorentzian peak-to-peak line widths ‫ܤߜ‬ NOgas of the corresponding NO gas signals were determined by simulation. 20 The variation of ݊ RT with these line widths follows in a good approximation a linear relation ݊ RT ൫ߜ‫ܤ‬ NOgas ൯ (S3). This relation was used to estimate from ‫ܤߜ‬ NOgas at T ≈ 125 K the mol numbers of desorbed NO gas in samples F62, F860, R60 and R800.…”

“…Test measurements performed after a longer time (up to 15 minutes) showed no spectral changes indicating that thermal equilibrium was almost reached before each measurement.Simulations of the EPR signals were realised with the MATLAB toolbox EasySpin 32 which employs the exact diagonalization of the spin Hamiltonian matrix. The signal of NO gas was simulated as described recently 20. …”