Jet-Cooled Phosphorescence Excitation Spectrum of the T1(n,π*) ← S0 Transition of 4 H-Pyran-4-one

Abstract

The 4H-pyran-4-one (4PN) molecule is a cyclic conjugated enone with spectroscopically accessible singlet and triplet (n,π*)excited states. Vibronic spectra of 4PN provide a stringent test of electronic-structure calculations, through comparison of predicted vs measured vibrational frequencies in the excited state. We report here the T₁(n,π*) ← S₀ phosphorescence excitation spectrum of 4PN, recorded under the cooling conditions of a supersonic free-jet expansion. The jet cooling has eliminated congestion appearing in previous room-temperature measurements of the T₁ ← S₀ band system and has enabled us to determine precise fundamental frequencies for seven vibrational modes of the molecule in its T₁(n,π*) state. We have also analyzed the rotational contour of the 0₀⁰ band, obtaining experimental values for spin-spin and spin-rotation constants of the T₁(n,π*) state. We used the experimental results to test predictions from two commonly used computational methods, equation-of-motion excitation energies with dynamical correlation incorporated at the level of coupled cluster singles doubles (EOM-EE-CCSD) and time-dependent density functional theory (TDDFT). We find that each method predicts harmonic frequencies within a few percent of observed fundamentals, for in-plane vibrational modes. However, for out-of-plane modes, each method has specific liabilities that result in frequency errors on the order of 20-30%. The calculations have helped to identify a perturbation from the T₂(π,π*) state that leads to unexpected features observed in the T₁(n,π*) ← S₀ origin band rotational contour.

Figure 1

Structural formula of 4H-pyran-4-one (4PN), along with the molecule-fixed coordinate system used here. The molecule has a C_2v planar equilibrium geometry in its ground and T₁(n,π*) states.

Figure 2

Overview of the T₁(n,π*) ← S₀ phosphorescence excitation spectrum of 4PN, recorded using a supersonic free-jet expansion. The buffer gas was 5 atm of helium. The maximum of the 0₀⁰ origin band is located at 27293.2 cm^–1. Inset shows the origin band on an expanded horizontal scale.

Figure 3

Low-wavenumber region of the spectrum in Figure 2. Blue tie lines show assignments of vibronic cold bands (v″ = 0), and red tie lies show assignments of hot bands (v″ > 0). Franck–Condon factors (FCF) were computed using a procedure described in the text. Assignment labels without tie lines, shown above the spectrum, refer to FC-forbidden bands.

Figure 4

Continuation of the spectrum in Figure 3.

Figure 5

Continuation of the spectrum in Figure 4. The black trace is plotted using the same vertical scale as in Figures 3 and 4. The green trace is plotted on a contracted (× 1/4) vertical scale in order to show peaks near 800 and 900 cm^–1, which have maximum intensity.

Figure 6

Region of T₁(n,π*) ← S₀ phosphorescence excitation spectrum of 4PN, about 700 cm^–1 above the origin band. These traces show the suppression of S₁(n,π*) ← S₀ hot bands under colder helium expansion conditions.

Figure 7

Jet-cooled T₁(n,π*) ← S₀ phosphorescence excitation spectrum of 4PN, recorded using a helium backing pressure of 2 atm. The region of the 0₀⁰ band is shown. The band origin, listed in Table 4, was determined using a simulation procedure outlined in the text. Branch labels are given in the format ^ΔNΔJ, where the O-form branch corresponds to ΔN = −2; the P-form branch corresponds to ΔN = −1, etc.

Figure 8

Experimental spectrum of Figure 7 (black trace), along with simulation (red trace) produced using the molecular constants listed in Table 4. The simulation used a two-temperature model (see text) to describe the distribution of rotational levels in the ground state. The two temperatures were chosen to be 4 and 16 K, with weighting factors of 0.4 and 0.6, respectively.

Figure 9

Experimental spectrum of Figure 7 (black trace), along with simulation (red trace) produced using T₁(n,π*) inertial constants from an EOM-EE-CCSD calculation rather than TDDFT (PBE0).

Figure 10

Origin band of the 4PN T₁(n,π*) ← S₀ transition, simulated using a range of α values departing from its optimum (red trace) by +20% to −20%. Other molecular constants are those in Table 4. Simulations employing positive and negative deviations in α are shown in blue and green, respectively. Black trace is the experimental spectrum.

Figure 11

Isosurfaces of the canonical molecular orbitals for the lowest-energy electronic transition of 4PN. The value of |ψ| on each isosurface is 0.050 Å^–3/2.