Infrared Spectroscopy of Fluorenyl Cations at Cryogenic Temperatures

The notion of (anti)aromaticity is a successful concept in chemistry to explain the structure and stability of polycyclic hydrocarbons. Cyclopentadienyl and fluorenyl cations are among the most studied classical antiaromatic systems. In this work, fluorenyl cations are investigated by high-resolution gas-phase infrared spectroscopy in helium droplets. Bare fluorenyl cations are generated in the gas phase by electrospray ionization. After mass-to-charge selection, ions are captured in ultracold helium nanodroplets and probed by infrared spectroscopy using a widely tunable free-electron laser in the 600–1700 cm–1 range. The highly resolved cryogenic infrared spectra confirm, in combination with DFT computations, that all cations are present in their singlet states.


First Round of Reviewer Comments
Reviewer: 1

Comments to the Author
In this manuscript, the authors report the spectroscopy of three fluorenyl ca�ons (Fl+).Comparison with theory demonstrates that the ions are all in their singlet spin states.The authors also address whether these species are aroma�c or an�aroma�c.The work is presented well and can be published a�er the authors consider sugges�ons for making their paper a bit clearer.

1)
I had to look up pKR, so I would suggest they men�on this is the pKa of the side chain.
2) Page 1, second column, the authors state "These studies suggest that the fluorenyl ca�on is rather non-aroma�c."From the descrip�ons provided, it is unclear to me how these studies have shown that.Please elucidate further.

3)
Figure 2. I think the shading on the energies detracts from legibility and the font size here should be bigger.In part b, I would include the phenyl ring on the structure shown as this would beter illustrate the steric effects men�oned in the text.

5)
The agreement between theory and experiment is really quite good with the notable excep�ons of the bands near 1500 cm-1 for R = H and Ph.The authors really should provide some comment on what is going on here, especially for part a where there is an extra band observed but not predicted.

6)
At the end of the main paper, I am le� hanging.Do the authors conclude that these three Fl+ species are aroma�c, an�-aroma�c, or non-aroma�c?This could be stated either at the end of the second to last paragraph or in the "conclusion" paragraph.

7)
In the conclusion paragraph, the authors state "The structural characteriza�on of these elusive ions directly shows that the 9-phenyl and the 9hydroxy-9-fluorenyl ca�ons are substan�ally stabilized by their subs�tuent in contrast to the 9-fluorenyl ca�on."Earlier on this page, the authors do clarify why the phenyl subs�tuent provides stabiliza�on, but the paragraph for the OH subs�tuent does not provide any insight into the conclusion made.

8)
Computa�onal methods: If the authors performed calcula�ons at different levels of theory, please provide us with a quick synopsis of the results compared to the CAM-B3LYP results and why the later were chosen for comparison in the main text.

Reviewer: 2
Comments to the Author Greis et al. present new IR spectra of the gas-phase fluorenyl ca�on and two of its deriva�ves.The spectra are of very high quality and improve very significantly upon spectra of these species reported in the literature, thanks to the unique instrument constructed by von Helden and coworkers: the combina�on of a tunable IR free electron laser, a tandem mass spectrometer and a He-droplet molecular beam.I believe that this is a beau�ful demonstra�on of the state-of-the-art in gas-phase ion spectroscopy and how it can be applied to ques�ons of fundamental physical chemistry nature: the singlet versus triplet character of these molecular ions that are classified as an�-aroma�c in the Hückel defini�on.As such, I believe that this study deserves publica�on in JPCL.
My only concern is the slightly naïve tone of the introduc�on.Clearly, molecular theory has progressed beyond Hückel MO theory, even though its concepts are s�ll widely in use.For instance, it is well known that many poyaroma�cs, such as pyrene and coronene, that are an�-aroma�c in the Hückel sense, are stable molecules with a singlet electronic ground state.In fact, the authors make use of computa�onal methods (DFT) that are well beyond the Hückel approxima�on and that indeed clearly indicate a singlet ground state for the ca�onic systems addressed in this study.In that sense, the finding of a singlet ground state as presented here is not so surprising.Nonetheless, the spectroscopic evidence provided in this Leter is very elegant and extends such evidence to an�-aroma�c polyaroma�cs in ca�onic charge states.
Despite the somewhat naïve introduc�on, I do like the Hückel-based conceptual ra�onale for the observed singlet character of the ground state given towards the conclusion of the paper: the fluorenyl ca�on is regarded as two separate benzene moie�es, which are then both aroma�c in the Hückel sense.
In this regard, the manuscript is nicely �ed together.
A few small points that deserve aten�on: The Computa�onal sec�on men�ons anharmonic frequency calcula�ons that have been performed.Nonetheless, the main paper shows only results from scaled-harmonic calcula�ons.Why do expensive theory if you do not use it?Also, is the (uniform) scaling factor used here adopted from literature or is this the best fit?
The cap�on to Fig 2 men�ons that the energies and the structures are from results at the CAM-B3LYP... level.I then assume that the computed spectra are also obtained at that level or are they from one of the other calcula�ons men�oned in the Computa�onal Sec�on?
In the Experimental Sec�on, the nozzle temperature and/or approximate He cluster size could be men�oned as a key experimental parameter.
The descrip�on of the Fl+ spectrum in the last full paragraph on page 2 appears to skip over the most striking aspect of the spectra in Fig. 2a: the doublet experimental feature near 1600 versus the single band in the theore�cal spectrum.Any comment on this discrepancy?
End of page 2: the �lt angle of the Ph plane rela�ve to the Fl plane should be specified.Supposedly it is small, because otherwise it would disrupt the mesomeric pi-dona�on men�oned a paragraph ago, right?page 3, second column, line 3 from top: "615 at 1538" should be "615 and 1538" Author's Response to Peer Review Comments: Dear Professor: Thank you very much for forwarding us the reviewers' comments on our manuscript "Infrared Spectroscopy of Fluorenyl Cations at Cryogenic Temperatures" and the opportunity to submit a revised version.
First, we would like to thank the reviewers for their great effort with the evaluation of this manuscript.Their comments helped to significantly improve the manuscript.The revised manuscript is submitted as a Word file with changes highlighted in yellow.We address the reviewers' requests in the point-by-point response attached to this letter.
Thank you for considering our manuscript for publication in The Journal of Physical Chemistry Letters.
With kind regards,

Gert von Helden Kim Greis
Editorial Requests: 1) Author Affiliations: Please include postal codes/country in the author affiliations in the publication file(s).
#The postal codes and countries have been included for all authors.Furthermore, updated addresses of two of the authors have been provided.# 2) Author Affiliations: Please provide all affiliations in English.#The TOC graphic was resized as suggested and moved to the same page as the abstract.#--------------------------------

Reviewer 1:
In this manuscript, the authors report the spectroscopy of three fluorenyl cations (Fl+).
Comparison with theory demonstrates that the ions are all in their singlet spin states.The authors also address whether these species are aromatic or antiaromatic.The work is presented well and can be published after the authors consider suggestions for making their paper a bit clearer.
#We thank the reviewer for their very positive assessment of our manuscript!# 1) I had to look up pKR, so I would suggest they mention this is the pKa of the side chain.
#We agree with the reviewer that the denotation pKR is not very clear.Based on the cited reference, we decided to modify the sentence more descriptively to "solvolysis rates of the hydroxylated precursors" instead of using abbreviations.# 2) Page 1, second column, the authors state "These studies suggest that the fluorenyl cation is rather non-aromatic."From the descriptions provided, it is unclear to me how these studies have shown that.Please elucidate further.
#We changed the sentence to "The reported NMR-shifts, as well as computed nucleusindependent chemical shift values do not support antiaromaticity in the case of fluorenyl cation derivatives" to indicate based on which observations this conclusion is drawn.# 3) Figure 2. I think the shading on the energies detracts from legibility and the font size here should be bigger.In part b, I would include the phenyl ring on the structure shown as this would better illustrate the steric effects mentioned in the text.
#As suggested by the reviewer we generally increased the font sizes in figure 2 and removed the shading on the energies.Additionally, we included the phenyl ring including its tilted angle in the structure in Figure 2b.# 4) Page 3, column 2. "… at 615 at 1538 cm-1,…" Is the second "at" supposed to be "and"?
#We thank the reviewer for spotting this typo!# 5) The agreement between theory and experiment is really quite good with the notable exceptions of the bands near 1500 cm-1 for R = H and Ph.The authors really should provide some comment on what is going on here, especially for part a where there is an extra band observed but not predicted.
#We agree with the reviewer that it is worthwhile mentioning the discrepancy between experiment and theory in Figures 2a and 2b.
For the pure fluorenyl cation in Figure 2a there are two intense absorption bands in the experiment at 1572 and 1583 cm -1 .In the computed spectrum at the CAM-B3LYP level of theory, there is mainly one frequency at 1591 cm -1 originating from coupled in-plane C=C stretches of the whole system.Calculations at this and other levels of theory shows that none of them can reproduce these two signals accurately.Even the anharmonic frequencies do not reproduce it.
However, there are significant differences in the predicted positions of absorption bands at different levels of theory (Figure S2).The reason for this discrepancy is unclear.We added the following sentences to the manuscript "There are two absorption bands at 1572 and 1583 cm -1 , whereas computed harmonic spectra at different levels of theory (Figure S2) mainly predict one matching band in this region.The origin of this discrepancy is unclear..".
For then phenylfluorenyl cation in Figure 2b, on the other hand, we believe that the discrepancy due to experiment and theory is mainly due to the employed level of theory.At other levels of theory, such as PBE0, as shown in the new Figure S3, it becomes apparent that the match of the feature around 1600 cm -1 is rather good.With the CAM-B3LYP functional the shape of the feature matches quite well, but it is shifted by ca.20 cm -1 .We added the following sentence to the manuscript: "The experimental features around 1580 cm -1 are shifted by ca.20 cm -1 compared to the computed spectrum at the CAM-B3LYP+D3/Def2-TZVPP level of theory of the singlet ion.
However, the position of these harmonic frequencies is strongly dependent on the employed level of theory, as showcased in Figure S3."# 6) At the end of the main paper, I am left hanging.Do the authors conclude that these three Fl+ species are aromatic, anti-aromatic, or non-aromatic?This could be stated either at the end of the second to last paragraph or in the "conclusion" paragraph.
#In the second last paragraph, we describe that a triplet state or an allylic structure would disrupt the aromaticity of the annelated benzyl rings.To this paragraph we added the following statement: "A categorization into aromatic or antiaromatic according to the Hückel rules is compelling but does not do justice to the complexity of this system.As previously reported, a classification into #Empirical scaling factors are necessary to match the harmonic frequencies to the experimentally obtained spectra.The uniform scaling factor of 0.965 has been frequently used for several publications from our lab in the last five years.When used, the harmonic spectra often match well with the experimentally obtained spectra, for several density functionals and basis sets.#

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All affiliations have been provided in English.The name Freie Universität Berlin is an official name that does not have an official counterpart in English.# 3) References: In both the main file and the supporting information, fix the style of all references to use JPCL formatting (check all references carefully).***JPC Letters reference formatting requires that journal references should contain: () around numbers; author names; article title (titles entirely in title case or entirely in lower case); abbreviated journal title (italicized); year (bolded); volume (italicized); and pages (first-last).Book references should contain author names; book title (in the same pattern); publisher; city; and year.Websites must include date of access.#Weadapted the references according to the editor's suggestions.#4) Supporting Information: Please number SI pages in the following format: "S1, S2…" #The SI page numbers were adapted as suggested.#5) TOC Graphic: Please resize the TOC graphic per journal guidelines (2 in x 2 in) and move to the correct position (on the same page as the abstract).