The reason for the selection of 350 K was that we were interested in studying the general behavior of species present in the system and (in) gaining a better understanding of the system's dynamics.
This is perfectly grammatical, but it is long and complex, and may not read optionally. It can be shortened by using "because" instead of 'the reason for".
- We selected 350 K because we were interested in studying the general behavior of species present in the system and in gaining a better understanding of the system's dynamics.
- Because we were interested in studying the general behavior of species present in the system and in gaining a better understanding of the system's dynamics, we selected (the) 350 K.
- We selected 350 K in order to study the general behavior of species present in the system and gain a better understanding of the system's dynamics.
The third form avoids the gerunds and may be clearer.
(I am not clear whether "350 K" is a model of equipment, or a temperature (350 degrees kelvin) at which the work was done, or what, so I am not sure if an article is needed in the second form.) T
"The reason for the selection of 350 K was that ..." is a passive form, and does not explicitly indict who did the selecting, but since "we" is specified later in the sentence, a form explicitly saying "we selected" is possible.
One possible complication is that the given sentence does not indicate why 350 K was a good, or perhaps even the best, selection for the stated purpose. Sod it doesn't really give the reason for the selection, only the goal to be achieved via that selection. I suppose this would be stated in nearby text in the full document. But this makes the sentence feel more awkward in isolation, at least to me.
The reason for this selection was providing more ...
could be recast as
This selection was made to provide more ...
or
We made this selection to provide more ...