Result of custom SQL string

I have following SQL. It adds a column specified as parameter. The section to add column is fine, I need it extended so that only adds column if it does not exist already.
The line
if exec sp_executesql @checkExists
fails to run - not sure what the syntax should be here.
Thanks in advance  for help

ALTER PROCEDURE [dbo].[CreateColumn]
	-- Add the parameters for the stored procedure here
	@NewColumnName varchar(300)
AS
BEGIN
	-- SET NOCOUNT ON added to prevent extra result sets from
	-- interfering with SELECT statements.
	SET NOCOUNT ON;
 
	-- add column if it does not already exist
	declare @checkExists nvarchar(3000)
	set @checkExists = 'IF NOT EXISTS ( SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE ' +
	                   'TABLE_NAME = HistorianData AND COLUMN_NAME = ' + @NewColumnName + ')'
	-- exec SQL to check if column exists...fails here
	if exec sp_executesql @checkExists
		BEGIN
			-- add new column
			declare @n nvarchar(3000)
 
			set @n = 'ALTER TABLE HistorianData
						ADD ' + @NewColumnName + ' FLOAT'
 
			exec sp_executesql @n
		END
END

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