I think the issue is with your syntax. You can't just write -vec like its algebra, you have to write -1 * vec, as below:
dplyr::tibble(power = c(9.5, 9.6, 9.7)) |>
dplyr::mutate(out1 = 2 ^ power,
out2 = 2 ^ (-1 * power))
#> # A tibble: 3 × 3
#> power out1 out2
#> <dbl> <dbl> <dbl>
#> 1 9.5 724. 0.00138
#> 2 9.6 776. 0.00129
#> 3 9.7 832. 0.00120
Created on 2022-06-30 by the reprex package (v2.0.1)