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%% This is a (brief) model paper using the achemso class %% This is a (brief) model paper using the achemso class
%% The document class accepts keyval options, which should include %% The document class accepts keyval options, which should include
@ -245,7 +247,9 @@ For (H$_2$O)$_6$UH$^+$ cluster, the neutral uracil molecule loss proportion is 1
Figure \ref{fig:neutralUloss} displays the neutral uracil molecule loss proportion 17.4\% of n=7, which is from the second lowest-energy isomer of (H$_2$O)$_7$UH$^+$ cluster (see 7b in Figure \ref{fig:6ato7b}) (W-H-U form). As displayed in Figure \ref{fig:6ato7b}, the proportion of the first lowest-energy isomer 7a (W-H form) of (H$_2$O)$_7$UH$^+$ cluster is 27.7\% shown in Table \ref{tab:table1}. The proportion of 7b, 17.4\% is close to the experiment result 15.8\%. From the dynamics collision simulations process for (H$_2$O)$_7$UH$^+$ cluster, the direct dissociation is dominant. It implies the neutral uracil loss proportion is determined by the initial configuration of the cluster. The relative energy of 7a and 7b calculated at MP2/Def2TZVP level is only 0.3 kcal$\cdot$mol$^{-1}$. This indicates the exist of isomer 7b that the excess proton is between the water cluster and uracil plays a dominant role after collision. Figure \ref{fig:neutralUloss} displays the neutral uracil molecule loss proportion 17.4\% of n=7, which is from the second lowest-energy isomer of (H$_2$O)$_7$UH$^+$ cluster (see 7b in Figure \ref{fig:6ato7b}) (W-H-U form). As displayed in Figure \ref{fig:6ato7b}, the proportion of the first lowest-energy isomer 7a (W-H form) of (H$_2$O)$_7$UH$^+$ cluster is 27.7\% shown in Table \ref{tab:table1}. The proportion of 7b, 17.4\% is close to the experiment result 15.8\%. From the dynamics collision simulations process for (H$_2$O)$_7$UH$^+$ cluster, the direct dissociation is dominant. It implies the neutral uracil loss proportion is determined by the initial configuration of the cluster. The relative energy of 7a and 7b calculated at MP2/Def2TZVP level is only 0.3 kcal$\cdot$mol$^{-1}$. This indicates the exist of isomer 7b that the excess proton is between the water cluster and uracil plays a dominant role after collision.
From the determination conducted by Zamith’s \textit{et al.}, the neutral uracil loss proportion starts to decrease from n=9, this absorbed us to perform the dynamics collision simulation of big cluster (H$_2$O)$_{12}$UH$^+$ as an example to explore why it has this change.\cite{Braud2019} For cluster (H$_2$O)$_{12}$UH$^+$, Figure \ref{fig:neutralUloss} displays the neutral uracil molecule loss proportion, 14.0\%, which is from the XX lowest-energy isomer (W-H-U form) (see 12x in Figure \ref{fig:12ad}). For the first lowest-energy isomer 12a that the excess proton is completely on the water cluster (W-H form), the proportion is 17.8\%. According to the dynamics collision simulations process of (H$_2$O)$_{12}$UH$^+$ cluster, both the direct dissociation and with structural rearrangements before dissociation are dominant after collision. This can be illustrated through the data of neutral uracil molecule loss proportion. Through the comparison for the proportion of 5a (34.9\%), 6a (36.6\%), 7a (27.7\%), and 12a (17.8\%), whose excess proton is on the water cluster and far from uracil, it’s clear that with the increase of the number of water molecules in mixed clusters the evaporation of neutral uracil decreases. Especially for big cluster (H$_2$O)$_{12}$UH$^+$, the neutral uracil loss has a significant reduction, this implies it has a structure rearrangement before dissociation. For 6f and 12x, the excess proton is bounded to water cluster but close to an oxygen atom of uracil, the loss of neutral uracil of 12x is lower than it of 6f, which also indicates the structure rearrangements occur in 12x. The proportion of 12x, 14.0\%, is closer to the one, 12.2\%, in experiment than 17.8\% of 12a. The relative energy of 12a and 12x calculated at MP2/Def2TZVP level is XX kcal$\cdot$mol$^{-1}$, it shows that the collision of 12x with Ar plays a dominant role. From the determination conducted by Zamith’s \textit{et al.}, the neutral uracil loss proportion starts to decrease from n=9,\cite{Braud2019} this attracted us to perform the dynamics collision simulation of big cluster (H$_2$O)$_{12}$UH$^+$ as an example to explore why it has this change. For cluster (H$_2$O)$_{12}$UH$^+$, Figure \ref{fig:neutralUloss} displays the neutral uracil molecule loss proportion, 14.0\%, which is from the sixth lowest-energy isomer. For the first lowest-energy isomer 12a (W-H form, see 12a in Figure \ref{fig:12ad}), the corresponding proportion is 6.1\%. The neutral uracil loss proportions of the second and fifth are 17.8\% and 3.7\%, separately. From 12a in Figure \ref{fig:12ad}, we can see the uracil in 12a is belong to the di-keto form (there is a hydrogen atom on each nitrogen of uracil), and the excess proton was separately by one water molecule from uracil, additionally, the uracil is surrounded by the water cluster, all of these lead the excess proton to go to the near oxygen atom of uracil. So the neutral uracil loss proportion of 12a is only 6.1\%. For 12b the excess proton is on the water cluster and very far from the uracil (W-H form, see 12b in Figure \ref{fig:12ad}), which leads to a ralative high neutral uracil loss proportion (17.8\%). For 12e in Figure \ref{fig:12ad} (W-H-U form), the distance between the excess proton \blue{HX and OY} is 1.575 Å, and the uracil is surrounded by the water cluster so the neutral uracil loss proportion is very low (3.7\%). For 12f (W-H-U form, the distance between the excess proton \blue{HX and OY} is 1.624 Å, and the uracil is not surrounded by the water cluster (see 12f in Figure \ref{fig:12ad}), which leads a higher neutral uracil loss proportion (14.0\%) than that of 12e. From the results of 12a, 12b, 12e, and 12f, it indicates for cluster (H$_2$O)$_{12}$UH$^+$, the initial configurations also play an important role for the dissociation products.
Through comparing the neutral uracil molecule loss proportion of different W-H form clusters: the proportion of 5a (34.9\%), 6a (36.6\%), 7a (27.7\%), and 12b (17.8\%), it’s clear that with the increase of the number of water molecules in mixed clusters the evaporation of neutral uracil decrease; especially for big cluster (H$_2$O)$_{12}$UH$^+$, the neutral uracil loss has a significant reduction, this implies it has a structure rearrangement prior to dissociation. For 6f and 12f (W-H-U form), the loss of neutral uracil of 12f is lower than it of 6f, which also indicates the structure rearrangements occur in 12f. The proportion of 12f, 14.0\%, is closer to the one, 12.2\%, in experiment than 17.8\% of 12a and the relative energy of 12a and 12f calculated at MP2/Def2TZVP level is only 2.4 kcal$\cdot$mol$^{-1}$, which shows that the collision of 12f with Ar plays a dominant role. So for (H$_2$O)$_{12}$UH$^+$ cluster, both the direct dissociation and with structural rearrangements prior to dissociation are dominant after collision.
From Table \ref{tab:table1}, we can see for clusters (H$_2$O)$_{n=3-4}$UH$^+$ with U-H form, the proportions of neutral uracil loss are about 0.0\%. For different isomers of clusters (H$_2$O)$_{n=5-7}$UH$^+$ with the W-H form, they have a relative high proportion. For clusters (H$_2$O)$_{n=4-7}$UH$^+$ with W-H-U form, they have a relative low proportion and the proportion decreases with distance between the excess proton and the adjacent oxygen atom of uracil. For cluster (H$_2$O)$_{12}$UH$^+$, when the excess proton has the W-H form localization, it also has a higher proportion of neutral uracil loss than the one when the excess proton is the W-H-U form. In general, for any cluster of (H$_2$O)$_{n=3-7, 12}$UH$^+$, the neutral uracil loss proportion has a direct relationship with the localization form of the excess proton: proportion (W-H form) $>$ proportion (W-H-U form) $>$ proportion (U-H form). In other words, it is obvious that the neutral uracil loss proportions of clusters (H$_2$O)$_{n=3-7, 12}$UH$^+$ are significantly affected by the initial configuration of the clusters. However, for cluster (H$_2$O)$_{12}$UH$^+$, even the excess proton is the W-H form, it didn’t have a too high proportion owing to the structural rearrangements that the excess proton goes back to uracil after collision of cluster (H$_2$O)$_{12}$UH$^+$ and Ar. It’s worth noticing that why cluster (H$_2$O)$_{12}$UH$^+$ has structural rearrangements prior to dissociation. We propose for cluster (H$_2$O)$_{12}$UH$^+$, there are more hydrogen bonds than the small clusters, which needs more energy to lose water molecules, so the proton has time to go back to uracil before dissociation. From Table \ref{tab:table1}, we can see for clusters (H$_2$O)$_{n=3-4}$UH$^+$ with U-H form, the proportions of neutral uracil loss are about 0.0\%. For different isomers of clusters (H$_2$O)$_{n=5-7}$UH$^+$ with the W-H form, they have a relative high proportion. For clusters (H$_2$O)$_{n=4-7}$UH$^+$ with W-H-U form, they have a relative low proportion and the proportion decreases with distance between the excess proton and the adjacent oxygen atom of uracil. For cluster (H$_2$O)$_{12}$UH$^+$, when the excess proton has the W-H form localization, it also has a higher proportion of neutral uracil loss than the one when the excess proton is the W-H-U form. In general, for any cluster of (H$_2$O)$_{n=3-7, 12}$UH$^+$, the neutral uracil loss proportion has a direct relationship with the localization form of the excess proton: proportion (W-H form) $>$ proportion (W-H-U form) $>$ proportion (U-H form). In other words, it is obvious that the neutral uracil loss proportions of clusters (H$_2$O)$_{n=3-7, 12}$UH$^+$ are significantly affected by the initial configuration of the clusters. However, for cluster (H$_2$O)$_{12}$UH$^+$, even the excess proton is the W-H form, it didn’t have a too high proportion owing to the structural rearrangements that the excess proton goes back to uracil after collision of cluster (H$_2$O)$_{12}$UH$^+$ and Ar. It’s worth noticing that why cluster (H$_2$O)$_{12}$UH$^+$ has structural rearrangements prior to dissociation. We propose for cluster (H$_2$O)$_{12}$UH$^+$, there are more hydrogen bonds than the small clusters, which needs more energy to lose water molecules, so the proton has time to go back to uracil before dissociation.