Tautomerism and electronic spectroscopy of protonated 1- and 2-aminonaphthalene

Abstract

Experimental and theoretical investigations of the excited states of protonated 1- and 2-aminonaphthalene are presented. The electronic spectra are obtained by laser induced photofragmentation of the ions captured in a cold ion trap. Using ab initio calculations, the electronic spectra can be assigned to different tautomers which have the proton on the amino group or on the naphthalene moiety. It is shown that the tautomer distribution can be varied by changing the electrospray source conditions, favoring either the most stable form in solution (amino protonation) or that in the gas phase (aromatic ring protonation). Calculations for larger amino-polyaromatics predict that these systems should behave as "proton sponges" i.e. have a proton affinity larger than 11 eV.

Selecting and identifying gas-phase protonation isomers of nicotineH+ using combined laser, ion mobility and mass spectrometry techniques

“…72 Although we note that it can be somewhat subjective to assign the "vertical transition energy" from an experimental spectrum, there is little doubt that CC2 offers significant improvement on transition energy predictions in many published studies of aromatic ions. 13,73 For the CAM- For PYRI-nicH + , a strong transition is predicted around 5.15 -5.19 eV which corresponds to the S3S0 transition for all methods and this is assigned to the major peak in the m/z 84 action spectrum in Figure 4 (both A and B). The higher energy transition centred around 5.5 eV in Figure 4 is assigned to the S4S0 for the CC2 method but assigned as the S5S0 for the TD-DFT methods.…”

“…28,33 It is well known that proton affinity is an oversimplified predictor for ESI generated gasphase protomer populations, and this is well evidenced by reports showing that protomer populations can vary drastically with ESI and source conditions. 3,[12][13][14][25][26] Furthermore, cases are reported that show predictions with different computational methods can scramble the order of proton affinities, thus making it tricky to confidently predict the most stable gasphase protomer. [13][14]49 Therefore, for mass spectrometry, experimental tools that detect and assign protonation isomers are vital for validation of computational methods.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] This can be an analytical problem when protomers dissociate to different product ions under activation, a phenomenon that can confound assignment by comparison to reference spectra. Differences in the photodissociation of protomers have been used to assign protomer populations, [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] although it is difficult to predict how the electronic spectroscopy and photodissociation will be affected by differences in protonation site. Hence, analysing the photodissociation of selected protomers, can provide insights into the effects of protonation on excited quantum states of ions while progressing towards the use of photo-dissociation as a method to disambiguate and assign protomers.…”

“…Previous studies have taken advantage of the differences in photoactivity of protomers and made assignments by comparing theoretical transition simulations to experimental results obtained by UV photodissociation action spectroscopy, [13][14][15][16][17][24][25] or IRMPD. [10][11][18][19][20][21][22][23] Dessent and co-workers have demonstrated the potential for moderate-resolution UV action spectra, coupled with TD-DFT analysis, to detect and assign protomers of nicotinamide and paminobenzoicacid.…”

“…However, it has been shown that theoretical results cannot reliably predict the relative protomer populations generated by ESI. For example, protomer populations can be shifted by changing ESI conditions [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and less stable isomers can be kinetically trapped by the ESI process. 4,6,9,[47][48] Furthermore, the relative energies of protomers can depend strongly on different levels of theory and basis sets, which can result in the prediction of different lowest energy protomers using different computational approaches.…”