Fig 1.
Compounds studied within this work.
Fig 2.
First derivative Q-band EPR spectra computed from experimental echo detected field-sweep spectra (black curves) for the set of FLP adducts.
Red and dotted blue curves are EasySpin [23] simulations considering different sets of parameters as follows: (a) simulations (red curves) considering “best fit” parameters from the previous X-band results [8]; (b) red curves—simulations based on the DFT-calculated parameters from Ref. [8]; blue curves—simulations based on the DFT-calculated parameters and subsequent adjustments of the g-tensor principal values, as shown in Table 1; and (c) simulations considering the best-fit parameters shown on the third column of Table 1, followed by adjustment of the anisotropic g-values (see Table 1). For samples 1 to 3 the Euler angles for the different tensors were taken from DFT calculations (Ref. [8]). For samples 4 to 6 DFT calculations were not performed and the set of Euler angles calculated for sample 1 was used in the simulations.
Table 1.
Hyperfine (A-), Quadrupole (Q-) and g-tensor principal values for the set of compounds studied in this work obtained by DFT calculations [8] and from X-band [8] and Q-band EPR analyses.
The conventions Axx < Ayy < Azz and gxx > gyy > gzz are followed. The parameters δA an ηA are calculated according to Eq (2). The DFT calculations were done on geometry-optimized structures from the gas phase on a TPSS-D3/def2-TZVP level, the A-tensors were calculated on a B3LYP/TZ2P level [8].
Fig 3.
2D-HYSCORE spectra recorded at a magnetic field strength of 1.23 T for the set of FLP samples 1–5.
The anti-diagonal dashed lines cross the diagonal at the Larmor frequencies for the isotopes related in the plots. The diagonal peaks at around 34 MHz and 42 MHz are artifacts from the spectrometer.
Fig 4.
2D-HYSCORE simulated spectra (left) compared with HYSCORE experimental spectrum for sample 3 (right).
(a) parameters obtained from DFT calculations [8], (b) parameters extracted from X-band EPR analysis [8], and (c) optimized parameters. The simulations were performed at Q-band frequencies and a magnetic field of 1.23 T. Table 1 shows the set of EPR parameters used in the simulations. The diagonal peaks at around 34 MHz and 42 MHz are artifacts from the spectrometer.
Table 2.
EPR interaction parameters used for the X-band ESEEM simulations for samples 2–5.
Aiso, δA and ηA are respectively the isotropic and anisotropy and asymmetry parameters of the 11B hyperfine coupling tensor, according to the notation given in Eq 2. α, β and γ are the Euler angles, according to notation from Ref. [16], and CQ and ηQ are the 11B nuclear electric quadrupole coupling constant and the EFG asymmetry parameter.
Fig 5.
Experimental Q-band ESEEM spectra for the samples 1–5 (black curves).
Red curves are EasySpin simulations considering “best fit” principal values of the interaction tensors obtained from the HYSCORE results (see Table 1) and Euler angles from DFT calculations [8]. The asterisk marks indicate frequency positions where spectrometer artifacts are present (narrow peaks).
Fig 6.
Simulated (red curves) and experimental (black curves) Q-band ESEEM spectra for sample 2.
The simulations consider different sets of nuclear species interacting with a single unpaired electron. (a) 10B, 11B, 14N and 31P nuclei (red curve); (b) 11B nucleus only; (c) 14N nucleus only; (d) 10B nucleus only; (e) 11B and 14N nuclei; (f) 10B and 14N nuclei; (g) 31P nucleus only; (h) 31P and 14N nuclei; (i) 31P and 10B nuclei. Spectra are internally normalized by the maximum intensity. The asterisk marks indicate frequency positions where spectrometer artifacts are present (narrow peaks). The blue dashed curve in (a) shows an optimized simulation including the interactions with all the nuclei and emphasizing the 14N contribution (see text).
Fig 7.
X-band cw-EPR spectrum of samples 1–3 (black curves, from bottom to top) and simulation (red curves) using the parameters obtained from Q-band HYSCORE and EDFS analyses.
Fig 8.
Simulated (red curves) and experimental (black curve) X-band ESEEM spectra for sample 5 obtained with τ = 250 ns.
The simulations consider different sets of nuclear species interacting with a single unpaired electron. (a) 10B, 11B, 14N, 31P, 19F, and 1H nuclei; (b) 11B nucleus only; (c) 10B nucleus only; (d) 14N nucleus only; and (e) 31P nucleus only. The peaks in the 1H and 19F Zeeman frequencies are labeled in the figure.
Fig 9.
Experimental (black curves) and simulated (red curves) X-band ESEEM spectra obtained with τ = 170 ns (a) and τ = 250 ns (b) for the sample 5 in the temperature range 100–300 K.
The simulations where performed considering isotropic hyperfine coupling tensors for 1H and 19F nuclei with Aiso = 1.8 MHz and 0.8 MHz respectively, and hyperfine and quadrupolar coupling interactions with 11B using the interaction parameters listed in Table 2.
Fig 10.
Isotropic 11B hyperfine coupling constant (a) and 11B quadrupolar coupling constant (b) of samples 2–5 as a function of temperature.
The parameters were obtained from simulations of the X-band ESEEM spectra. Dashed lines are drawn as guides for the eyes.